130+ Coffee Interview Questions Answers For Ii To Seven Twelvemonth Experienced Programmers
Last Updated Date: 2nd March 2018
Time is changing in addition to so is Java interviews. Gone are the days, when knowing the departure betwixt String in addition to StringBuffer tin terminate assist you lot to transcend through the 2nd circular of interview, questions are becoming to a greater extent than advanced in addition to interviewers are asking to a greater extent than deep questions. When I started my career, questions similar Vector vs Array and HashMap vs Hashtable were the most pop ones in addition to only memorizing them gives you lot a goodness adventure to do good inward interviews, but non anymore. Nowadays, you lot volition acquire questions from the areas where non many Java programmer looks e.g. NIO, patterns, sophisticated unit of measurement testing or those which are hard to master copy e.g. concurrency, algorithms, information structures in addition to coding.
Since I similar to explore interview questions, I receive got got this huge listing of questions with me, which contains lots in addition to lots of questions on unlike topics. I receive got been preparing this MEGA listing for quite some fourth dimension in addition to at nowadays It's cook to portion with you lot guys.
It contains interview questions non alone from classic topics similar threads, collections, equals in addition to hashcode, sockets but also from NIO, array, string, Java 8 in addition to much more.
It has questions for both entry-level Java programmers in addition to senior developers with years of experience. No affair whether you lot are a Java developer of 1, 2, 3, 4, 5, 6, 8, nine or fifty-fifty 10 years of experience, you lot volition let on something interesting inward this list. It contains questions which are super piece of cake to answer, in addition to also, a inquiry which is tricky plenty for fifty-fifty seasoned Java programmers. If you lot demand to a greater extent than questions in addition to so you lot tin terminate also depository fiscal establishment check out this Java Interview Guide: 200+ Interview Questions with Answers
Given the listing is long in addition to nosotros receive got questions from everywhere, it's imperative that answers must last short, concise in addition to crisp, no fluffing at all. So apart from this paragraph, you lot volition alone listen from me is the questions in addition to answers, no to a greater extent than context, no to a greater extent than feedback in addition to no to a greater extent than evaluation.
For that, I receive got already written spider web log posts, where you lot tin terminate let on my views on a particular question, e.g. why I similar that question, what makes them challenging in addition to what form of reply you lot should await from candidates.
This listing is a footling fleck unlike in addition to I encourage you lot to portion questions in addition to answers inward a similar way so that it should last piece of cake to revise. I promise this listing tin terminate last a keen utilisation for both interviewer in addition to candidates, the interviewer can, of course, position a footling multifariousness on questions to convey novelty in addition to surprise element, which is of import for a goodness interview. While a candidate, tin terminate expand in addition to examine their cognition well-nigh key areas of Java programming linguistic communication in addition to platform.
In 2018 in addition to inward coming years the focus volition last to a greater extent than on advanced concurrency concept, JVM internals, 32-bit vs 64-bit JVM, unit of measurement testing, in addition to create clean code. I am sure, i time you lot read through this MEGA listing of Java interview question, you lot should last able to do good on both telephonic in addition to human face upwardly to human face upwardly programming interviews.
Java NIO in addition to JVM internals questions are best examples of that. You tin terminate maintain pattern patterns also on the same listing but growing number of an experienced programmer are at nowadays good aware of GOF pattern patterns in addition to when to utilisation them. I receive got also worked hard to maintain this listing up-to-date to include what interviewers are asking inward 2018 in addition to what volition last their center focus on coming years. To give you lot an idea, this listing of Java interview questions includes next topics:
You guys are also lucky that nowadays at that spot are some goodness books available to ready for Java interviews, i of them which I especially let on useful in addition to interesting to read is Java Programming Interview Exposed past times Markham. It volition take you lot to some of the most of import topics for Java in addition to JEE interviews, worth reading fifty-fifty if you lot are non preparing for Java interview.
1) Can nosotros brand array volatile inward Java?
This is i of the tricky Java multi-threading questions you lot volition come across inward senior Java developer Interview. Yes, you lot tin terminate brand an array volatile inward Java but alone the reference which is pointing to an array, non the whole array. What I mean, if i thread changes the reference variable to points to some other array, that volition render a volatile guarantee, but if multiple threads are changing private array elements they won't last having happens earlier guarantee provided past times the volatile modifier.
2) Can volatile brand a non-atomic functioning to atomic?
This some other goodness inquiry I beloved to enquire on volatile, mostly every bit a follow-up of the previous question. This inquiry is also non piece of cake to reply because volatile is non well-nigh atomicity, but at that spot are cases where you lot tin terminate utilisation a volatile variable to brand the functioning atomic.
One representative I receive got seen is having a long acre inward your class. If you lot know that a long acre is accessed past times to a greater extent than than i thread e.g. a counter, a cost acre or anything, you lot improve transcend far volatile. Why? because reading to a long variable is non atomic inward Java in addition to done inward 2 steps, If i thread is writing or updating long value, it's possible for some other thread to come across one-half value (fist 32-bit). While reading/writing a volatile long or double (64 bit) is atomic.
3) What are practical uses of volatile modifier?
One of the practical utilisation of the volatile variable is to brand reading double in addition to long atomic. Both double in addition to long are 64-bit broad in addition to they are read inward 2 parts, get-go 32-bit get-go fourth dimension in addition to side past times side 32-bit 2nd time, which is non-atomic but volatile double in addition to long read is atomic inward Java. Another utilisation of the volatile variable is to render a retention barrier, only similar it is used inward Disrupter framework. Basically, Java Memory model inserts a write barrier after you lot write to a volatile variable in addition to a read barrier earlier you lot read it. Which means, if you lot write to volatile acre in addition to so it's guaranteed that whatever thread accessing that variable volition come across the value you lot wrote in addition to anything you lot did earlier doing that right into the thread is guaranteed to receive got happened in addition to whatever updated information values volition also last visible to all threads, because the retention barrier flushed all other writes to the cache.
4) What guarantee volatile variable provides? (answer)
volatile variables render the guarantee well-nigh ordering in addition to visibility e.g. volatile assignment cannot last re-ordered with other statements but inward the absence of whatever synchronization didactics compiler, JVM or JIT are gratis to reorder statements for improve performance. volatile also provides the happens-before guarantee which ensures changes made inward i thread is visible to others. In some cases volatile also render atomicity e.g. reading 64-bit information types similar long in addition to double are non atomic but read of volatile double or long is atomic.
5) Which i would last piece of cake to write? synchronization code for 10 threads or 2 threads?
In price of writing code, both volition last of same complexity because synchronization code is independent of a number of threads. Choice of synchronization though depends upon a number of threads because the number of thread nowadays to a greater extent than contention, so you lot transcend for advanced synchronization technique e.g. lock stripping, which requires to a greater extent than complex code in addition to expertise.
6) How do you lot telephone outcry upwardly wait() method? using if block or loop? Why? (answer)
wait() method should e'er last called inward loop because it's possible that until thread gets CPU to start running over again the status powerfulness non hold, so it's e'er improve to depository fiscal establishment check status inward loop earlier proceeding. Here is the criterion idiom of using hold off in addition to notify method inward Java:
7) What is imitation sharing inward the context of multi-threading?
imitation sharing is i of the well-known performance issues on multi-core systems, where each procedure has its local cache. imitation sharing occurs when threads on unlike processor modify variables that reside on same cache delineate every bit shown inward the next image:
False sharing is rattling hard to let on because the thread may last accessing completely unlike global variables that occur to last relatively unopen together inward memory. Like many concurrency issues, the primary way to avoid imitation sharing is careful code review in addition to aligning your information construction with the size of a cache line.
8) What is busy spin? Why should you lot utilisation it?
Busy spin is i of the technique to hold off for events without releasing CPU. It's often done to avoid losing information inward CPU cached which is lost if the thread is paused in addition to resumed inward some other core. So, if you lot are working on depression latency organisation where your club processing thread currently doesn't receive got whatever order, instead of sleeping or calling wait(), you lot tin terminate only loop in addition to and so over again depository fiscal establishment check the queue for novel messages. It's alone beneficial if you lot demand to hold off for a rattling small-scale amount of fourth dimension e.g. inward micro seconds or nano seconds. LMAX Disrupter framework, a high-performance inter-thread messaging library has a BusySpinWaitStrategy which is based on this concept in addition to uses a busy spin loop for EventProcessors waiting on the barrier.
9) How do you lot take thread dump inward Java?
You tin terminate take a thread dump of Java application inward Linux past times using kill -3 PID, where PID is the procedure id of Java process. In Windows, you lot tin terminate press Ctrl + Break. This volition instruct JVM to impress thread dump inward criterion out or err in addition to it could transcend to console or log file depending upon your application configuration. If you lot receive got used Tomcat in addition to so when
10) is Swing thread-safe? (answer)
No, Swing is non thread-safe. You cannot update Swing components e.g. JTable, JList or JPanel from whatever thread, inward fact, they must last updated from GUI or AWT thread. That's why swings render invokeAndWait() in addition to invokeLater() method to asking GUI update from whatever other threads. This methods position update asking inward AWT threads queue in addition to tin terminate hold off till update or render straightaway for an asynchronous update. You tin terminate also depository fiscal establishment check the detailed reply to larn more.
11) What is a thread local variable inward Java? (answer)
Thread-local variables are variables confined to a thread, its similar thread's ain re-create which is non shared betwixt multiple threads. Java provides a ThreadLocal degree to back upwardly thread-local variables. It's i of the many ways to accomplish thread-safety. Though last careful piece using thread local variable inward manged environs e.g. with spider web servers where worker thread out lives whatever application variable. Any thread local variable which is non removed i time its piece of work is done tin terminate potentially displace a retention leak inward Java application.
12) Write wait-notify code for producer-consumer problem? (answer)
Please come across the reply for a code example. Just call upwardly to telephone outcry upwardly wait() in addition to notify() method from synchronized block in addition to examine waiting for status on the loop instead of if block.
13) Write code for thread-safe Singleton inward Java? (answer)
Please come across the reply for a code representative in addition to stair past times stair guide to creating thread-safe singleton degree inward Java. When nosotros say thread-safe, which agency Singleton should rest singleton fifty-fifty if initialization occurs inward the representative of multiple threads. Using Java enum every bit Singleton degree is i of the easiest ways to create a thread-safe singleton inward Java.
14) The departure betwixt slumber in addition to hold off inward Java? (answer)
Though both are used to interruption currently running thread, sleep() is genuinely meant for brusk interruption because it doesn't release lock, piece wait() is meant for conditional hold off in addition to that's why it release lock which tin terminate in addition to so last acquired past times some other thread to alter the status on which it is waiting.
15) What is an immutable object? How do you lot create an Immutable object inward Java? (answer)
Immutable objects are those whose Earth cannot last changed i time created. Any modification volition number inward a novel object e.g. String, Integer, and other wrapper class. Please come across the reply for stair past times stair guide to creating Immutable degree inward Java.
16) Can nosotros create an Immutable object, which contains a mutable object?
Yes, its possible to create an Immutable object which may incorporate a mutable object, you lot only demand to last a footling fleck careful non to portion the reference of the mutable component, instead, you lot should render a re-create of it if you lot receive got to. Most mutual representative is an Object which incorporate the reference of java.util.Date object.
17) What is the right information type to stand upwardly for a cost inward Java? (answer)
BigDecimal if retention is non a delineate of piece of work organisation in addition to Performance is non critical, otherwise double with predefined precision.
18) How do you lot convert bytes to String? (answer)
you lot tin terminate convert bytes to the string using string constructor which accepts byte[], only brand certainly that right grapheme encoding otherwise platform's default grapheme encoding volition last used which may or may non last same.
19) How do you lot convert bytes to long inward Java? (answer)
This questions if for you lot to reply :-)
20) Can nosotros cast an int value into byte variable? what volition occur if the value of int is larger than byte?
Yes, nosotros tin terminate cast but int is 32 fleck long inward coffee piece byte is 8 fleck long inward coffee so when you lot cast an int to byte higher 24 bits are lost in addition to a byte tin terminate alone concur a value from -128 to 128.
21) There are 2 classes B extends Influenza A virus subtype H5N1 in addition to C extends B, Can nosotros cast B into C e.g. C = (C) B; (answer)
22) Which degree contains clone method? Cloneable or Object? (answer)
java.lang.Cloneable is marking interface in addition to doesn't incorporate whatever method clone method is defined inward the object class. It is also knowing that clone() is a native method agency it's implemented inward C or C++ or whatever other native language.
23) Is ++ operator is thread-safe inward Java? (answer)
No it's non a thread rubber operator because its involve multiple instructions similar reading a value, incriminating it in addition to storing it dorsum into retention which tin terminate last overlapped betwixt multiple threads.
24) Difference betwixt a = a + b in addition to a += b ? (answer)
The += operator implicitly cast the number of add-on into the type of variable used to concur the result. When you lot add together 2 integral variable e.g. variable of type byte, short, or int in addition to so they are get-go promoted to int in addition to them add-on happens. If number of add-on is to a greater extent than than maximum value of a in addition to so a + b volition give compile fourth dimension fault but a += b volition last ok every bit shown below
25) Can I shop a double value inward a long variable without casting? (answer)
No, you lot cannot shop a double value into a long variable without casting because the attain of double is to a greater extent than that long in addition to you lot nosotros demand to type cast. It's non dificult to reply this inquiry but many develoepr acquire it wrong due to confusion on which i is bigger betwixt double in addition to long inward Java.
26) What volition this render 3*0.1 == 0.3? truthful or false? (answer)
This is i of the genuinely tricky questions. Out of 100, alone five developers answered this inquiry in addition to alone of them receive got explained the concept correctly. The brusk reply is imitation because some floating dot numbers tin terminate non last represented exactly.
27) Which i volition take to a greater extent than memory, an int or Integer? (answer)
An Integer object volition take to a greater extent than retention an Integer is the an object in addition to it store meta information overhead well-nigh the object in addition to int is primitive type so its takes less space.
28) Why is String Immutable inward Java? (answer)
One of my favorite Java interview question. The String is Immutable inward coffee because coffee designer thought that string volition last heavily used in addition to making it immutable allow some optimization piece of cake sharing same String object betwixt multiple clients. See the link for the to a greater extent than detailed answer. This is a keen inquiry for Java programmers with less experience every bit it gives them nutrient for thought, to think well-nigh how things industrial plant inward Java, what Jave designers powerfulness receive got thought when they created String degree etc.
29) Can nosotros utilisation String inward the switch case? (answer)
Yes from Java seven onward nosotros tin terminate utilisation String inward switch representative but it is only syntactic sugar. Internally string hash code is used for the switch. See the detaiedl reply for to a greater extent than explanation in addition to discussion.
30) What is constructor chaining inward Java? (answer)
When you lot telephone outcry upwardly i constructor from other than it's known every bit constructor chaining inward Java. This happens when you lot receive got multiple, overloaded constructor inward the class.
31) What is the size of int inward 64-bit JVM?
The size of an int variable is constant inward Java, it's e'er 32-bit irrespective of platform. Which agency the size of primitive int is same inward both 32-bit in addition to 64-bit Java virtual machine.
32) The departure betwixt Serial in addition to Parallel Garbage Collector? (answer)
Even though both the series in addition to parallel collectors displace a stop-the-world interruption during Garbage collection. The chief departure betwixt them is that a series collector is a default copying collector which uses alone i GC thread for garbage collection piece a parallel collector uses multiple GC threads for garbage collection.
33) What is the size of an int variable inward 32-bit in addition to 64-bit JVM? (answer)
The size of int is same inward both 32-bit in addition to 64-bit JVM, it's e'er 32 bits or four bytes.
34) Influenza A virus subtype H5N1 departure betwixt WeakReference in addition to SoftReference inward Java? (answer)
Though both WeakReference in addition to SoftReference helps garbage collector in addition to retention efficient, WeakReference becomes eligible for garbage collection every bit presently every bit in conclusion strong reference is lost but SoftReference fifty-fifty thought it tin terminate non preclude GC, it tin terminate delay it until JVM absolutely demand memory.
35) How do WeakHashMap works? (answer)
WeakHashMap industrial plant similar a normal HashMap but uses WeakReference for keys, which agency if the key object doesn't receive got whatever reference in addition to so both key/value mapping volition transcend eligible for garbage collection.
36) What is -XX:+UseCompressedOops JVM option? Why utilisation it? (answer)
When you lot transcend migrate your Java application from 32-bit to 64-bit JVM, the heap requirement of a abrupt increases, almost double, due to increasing size of ordinary object pointer from 32 fleck to 64 bit. This also adversely impact how much information you lot tin terminate maintain inward CPU cache, which is much smaller than memory. Since chief motivation for moving to 64-bit JVM is to specify large heap size, you lot tin terminate salve some retention past times using compressed OOP. By using -XX:+UseCompressedOops, JVM uses 32-bit OOP instead of 64-bit OOP.
37) How do you lot let on if JVM is 32-bit or 64-bit from Java Program? (answer)
You tin terminate let on that past times checking some organisation properties like sun.arch.data.model or os.arch
38) What is the maximum heap size of 32-bit in addition to 64-bit JVM? (answer)
Theoretically, the maximum heap retention you lot tin terminate assign to a 32-bit JVM is 2^32 which is 4GB but practically the bound is much smaller. It also varies betwixt operating systems e.g. shape 1.5GB inward Windows to almost 3GB inward Solaris. 64-bit JVM allows you lot to specify larger heap size, theoretically 2^64 which is quite large but practically you lot tin terminate specify heap infinite upwardly to 100GBs. There are fifty-fifty JVM e.g. Azul where heap infinite of M gigs is also possible.
39) What is the departure betwixt JRE, JDK, JVM in addition to JIT? (answer)
JRE stands for Java run-time in addition to it's required to run Java application. JDK stands for Java evolution kit in addition to provides tools to railroad train Java computer program e.g. Java compiler. It also contains JRE. The JVM stands for Java virtual machine in addition to it's the procedure responsible for running Java application. The JIT stands for Just In Time compilation in addition to helps to boost the performance of Java application past times converting Java byte code into native code when the crossed certainly threshold i.e. mainly hot code is converted into native code.
40) Explain Java Heap infinite in addition to Garbage collection? (answer)
When a Java procedure is started using coffee command, retention is allocated to it. Part of this retention is used to create heap space, which is used to allocate retention to objects whenever they are created inward the program. Garbage collection is the procedure within JVM which reclaims retention from dead objects for time to come allocation.
41) Can you lot guarantee the garbage collection process? (answer)
No, you lot cannot guarantee the garbage collection, though you lot tin terminate brand a asking using System.gc() or Runtime.gc() method.
42) How do you lot let on retention usage from Java program? How much percentage of the heap is used?
You tin terminate utilisation retention related methods from java.lang.Runtime degree to acquire the gratis memory, total retention in addition to maximum heap retention inward Java. By using these methods, you lot tin terminate let on out how many percents of the heap is used in addition to how much heap infinite is remaining. Runtime.freeMemory() render amount of gratis retention inward bytes, Runtime.totalMemory() returns total retention inward bytes in addition to Runtime.maxMemory() returns maximum retention inward bytes.
43) What is the departure betwixt stack in addition to heap inward Java? (answer)
Stack in addition to heap are unlike retention areas inward the JVM in addition to they are used for unlike purposes. The stack is used to concur method frames in addition to local variables piece objects are e'er allocated retention from the heap. The stack is commonly much smaller than heap retention in addition to also didn't shared betwixt multiple threads, but heap is shared with all threads inward JVM.
The a = b does object reference matching if both a in addition to b are an object in addition to alone render truthful if both are pointing to the same object inward the heap space, on the other hand, a.equals(b) is used for logical mapping in addition to its expected from an object to override this method to render logical equality. For example, String degree overrides this equals() method so that you lot tin terminate compare 2 Strings, which are the unlike object but contains same letters.
45) What is a.hashCode() used for? How is it related to a.equals(b)? (answer)
hashCode() method returns an int hash value corresponding to an object. It's used inward hash based collection classes e.g Hashtable, HashMap, LinkedHashMap in addition to so on. It's rattling tightly related to equals() method. According to Java specification, 2 objects which are equal to each other using equals() method must receive got same hash code.
46) Difference betwixt final, finalize in addition to finally? (answer)
The in conclusion is a modifier which you lot tin terminate apply to variable, methods in addition to classes. If you lot brand a variable in conclusion it agency its value cannot last changed i time initialized. finalize is a method, which is called only earlier an object is a garbage collected, giving it in conclusion adventure to resurrect itself, but the telephone outcry upwardly to finalize is non guaranteed. finally is a keyword which is used inward exception treatment along with endeavour in addition to catch. the finally block is e'er executed irrespective of whether an exception is thrown from endeavour block or not.
47) What is a compile fourth dimension constant inward Java? What is the endangerment of using it?
populace static in conclusion variables are also known every bit a compile fourth dimension constant, the populace is optional there. They are replaced with actual values at compile fourth dimension because compiler know their value up-front in addition to also knows that it cannot last changed during run-time. One of the occupation with this is that if you lot happened to utilisation a populace static in conclusion variable from some in-house or 3rd political party library in addition to their value changed after than your customer volition even so last using quondam value fifty-fifty after you lot deploy a novel version of JARs. To avoid that, brand certainly you lot compile your computer program when you lot upgrade dependency JAR files.
48) The departure betwixt List, Set, Map, in addition to Queue inward Java? (answer)
The listing is an ordered collection which allows duplicate. It also has an implementation which provides constant fourth dimension index based access, but that is non guaranteed past times List interface. Set is unordered collection which
49) Difference betwixt poll() in addition to remove() method?
Both poll() in addition to remove() take out the object from the Queue but if poll() fails in addition to so it returns zero but if take away fails it throws Exception.
50) The departure betwixt LinkedHashMap in addition to PriorityQueue inward Java? (answer)
PriorityQueue guarantees that lowest or highest priority chemical element e'er rest at the caput of the queue, but LinkedHashMap maintains the club on which elements are inserted. When you lot iterate over a PriorityQueue, iterator doesn't guarantee whatever club but iterator of LinkedHashMap does guarantee the club on which elements are inserted.
51) Difference betwixt ArrayList in addition to LinkedList inward Java? (answer)
The obvious departure betwixt them is that ArrrayList is backed past times array information structure, supprots random access in addition to LinkedList is backed past times linked listing information construction in addition to doesn't supprot random access. Accessing an chemical element with the index is O(1) inward ArrayList but its O(n) inward LinkedList. See the reply for to a greater extent than detailed discussion.
52) What is a pair of ways that you lot could sort a collection? (answer)
You tin terminate either utilisation the Sorted collection similar TreeSet or TreeMap or you lot tin terminate sort using the ordered collection similar a listing in addition to using Collections.sort() method.
53) How do you lot impress Array inward Java? (answer)
You tin terminate impress an array past times using the Arrays.toString() in addition to Arrays.deepToString() method. Since array doesn't implement toString() past times itself, only passing an array to System.out.println() volition non impress its contents but Arrays.toString() volition impress each element.
There are many differences betwixt these 2 classes, some of them are following:
a) Hashtable is a legacy degree in addition to nowadays from JDK 1, HashMap was added later.
b) Hashtable is synchronized in addition to slower but HashMap is non synchronized in addition to faster.
c) Hashtable doesn't allow zero keys but HashMap allows i zero key.
See the reply for to a greater extent than differences betwixt HashMap in addition to Hashtable inward Java.
57) How HashSet industrial plant internally inward Java? (answer)
HashSet is internally implemented using an HashMap. Since a Map needs key in addition to value, a default value is used for all keys. Similar to HashMap, HashSet doesn't allow duplicate keys in addition to alone i zero key, I hateful you lot tin terminate alone shop i zero object inward HashSet.
58) Write code to take away elements from ArrayList piece iterating? (answer)
Key hither is to depository fiscal establishment check whether candidate uses ArrayList's remove() or Iterator's remove(). Here is the sample code which uses right way o take away elements from ArrayList piece looping over in addition to avoids ConcurrentModificationException.
59) Can I write my ain container degree in addition to utilisation it inward the for-each loop?
Yes, you lot tin terminate write your ain container class. You demand to implement the Iterable interface if you lot desire to loop over advanced for loop inward Java, though. If you lot implement Collection in addition to so you lot past times default acquire that property.
60) What is default size of ArrayList in addition to HashMap inward Java? (answer)
As of Java seven now, default size of ArrayList is 10 in addition to default capacity of HashMap is 16, it must last powerfulness of 2. Here is code snippet from ArrayList and HashMap degree :
61) Is it possible for 2 unequal objects to receive got the same hashcode?
Yes, 2 unequal objects tin terminate receive got same hashcode that's why collision occur inward a hashmap.
the equal hashcode contract alone says that 2 equal objects must receive got the same hashcode it doesn't say anything well-nigh the unequal object.
62) Can 2 equal object receive got the unlike hash code?
No, thats non possible according to hash code contract.
63) Can nosotros utilisation random numbers inward the hashcode() method? (answer)
No, because hashcode of an object should last e'er same. See the reply to learning to a greater extent than well-nigh things to call upwardly piece overriding hashCode() method inward Java.
64) What is the departure betwixt Comparator in addition to Comparable inward Java? (answer)
The Comparable interface is used to define the natural club of object piece Comparator is used to define custom order. Comparable tin terminate last e'er one, but nosotros tin terminate receive got multiple comparators to define customized club for objects.
65) Why you lot demand to override hashcode, when you lot override equals inward Java? (answer)
Because equals receive got code contract mandates to override equals in addition to hashcode together .since many container degree similar HashMap or HashSet depends on hashcode in addition to equals contract.
66) In my Java program, I receive got iii sockets? How many threads I volition demand to handgrip that?
67) How do you lot create ByteBuffer inward Java?
68) How do you lot write in addition to read from ByteBuffer inward Java?
69) Is Java BIG endian or LITTLE endian?
70) What is the byte club of ByteBuffer?
71) The departure betwixt direct buffer in addition to non-direct buffer inward Java? (answer)
72) What is the retention mapped buffer inward Java? (answer)
73) What is TCP NO DELAY socket option?
74) What is the departure betwixt TCP in addition to UDP protocol? (answer)
75) The departure betwixt ByteBuffer in addition to StringBuffer inward Java? (answer)
76) What best practices you lot follow piece writing multi-threaded code inward Java? (answer)
Here are pair of best practices which I follow piece writing concurrent code inward Java:
a) Always call your thread, this volition assist inward debugging.
b) minimize the reach of synchronization, instead of making whole method synchronized, alone critical department should last synchronized.
c) prefer volatile over synchronized if you lot tin terminate can.
e) utilisation higher flat concurrency utilities instead of waitn() in addition to notify for inter thread communication e.g. BlockingQueue, CountDownLatch in addition to Semeaphore.
e) Prefer concurrent collection over synchronized collection inward Java. They render improve scalability.
77) Tell me few best practices you lot apply piece using Collections inward Java? (answer)
Here are pair of best practices I follow piece using Collectionc classes from Java:
a) Always utilisation the right collection e.g. if you lot demand non-synchronized listing in addition to so utilisation ArrayList in addition to non Vector.
b) Prefer concurrent collection over synchronized collection because they are to a greater extent than scalable.
c) Always utilisation interface to a stand upwardly for in addition to access a collection e.g. utilisation List to shop ArrayList, Map to shop HashMap in addition to so on.
d) Use iterator to loop over collection.
e) Always utilisation generics with collection.
78) Can you lot tell us at to the lowest degree five best exercise you lot utilisation piece using threads inward Java? (answer)
This is similar to the previous inquiry in addition to you lot tin terminate utilisation the reply given there. Particularly with thread, you lot should:
a) call your thread
b) maintain your task in addition to thread separate, utilisation Runnable or Callable with thread puddle executor.
c) utilisation thread pool
d) utilisation volatile to call for compiler well-nigh ordering, visibility, in addition to atomicity.
e) avoid thread local variable because wrong utilisation of ThreadLocal degree inward Java tin terminate create a retention leak.
Look at that spot are many best practices in addition to I give extra points to the developer which convey something new, something fifty-fifty I don't know. I brand certainly to enquire this inquiry to Java developers of 8 to 10 years of experience only to guess his hands on experience in addition to knowledge.
79) Name five IO best practices? (answer)
IO is rattling of import for performance of your Java application. Ideally you lot should avoid IO inward critical path of your application. Here are pair of Java IO best practices you lot tin terminate follow:
80) Name five JDBC best practices your follow? (answer)
Another goodness Java best practices for experienced Java developer of seven to 8 years experience. Why it's important? because they are the ones which laid the tendency inward the code in addition to educate junior developers. There are many best practices in addition to you lot tin terminate call every bit per your confort in addition to conviniece. Here are some of the to a greater extent than mutual ones:
a) utilisation batch declaration for inserting in addition to updating data.
b) utilisation PreparedStatement to avoid SQL exception in addition to improve performance.
c) utilisation database connective pool
d) access resultset using column call instead of column indexes.
e) Don't generate dynamic SQL past times concatenating String with user input.
81) Name pair of method overloading best practices inward Java? (answer)
Here are some best practices you lot tin terminate follow piece overloading a method inward Java to avoid confusion with auto-boxing:
a) Don't overload method where i accepts int in addition to other accepts Integer.
b) Don't overload method where number of declaration is same in addition to alone club of declaration is different.
c) Use varargs after overloaded methods has to a greater extent than than five arguments.
82) Does SimpleDateFormat is rubber to utilisation inward the multi-threaded program? (answer)
No, unfortunately, DateFormat in addition to all its implementations including SimpleDateFormat is non thread-safe, thus should non last used inward the multi-threaded computer program until external thread-safety measures are applied e.g. confining SimpleDateFormat object into a ThreadLocal variable. If you lot don't do that, you lot volition acquire an wrong number piece parsing or formatting dates inward Java. Though, for all practical appointment fourth dimension purpose, I highly recommend joda-time library.
83) How do you lot format a appointment inward Java? e.g. inward the ddMMyyyy format? (answer)
You tin terminate either utilisation SimpleDateFormat degree or joda-time library to format appointment inward Java. DateFormat degree allows you lot to format appointment on many pop formats. Please come across the reply for code samples to format appointment into unlike formats e.g. dd-MM-yyyy or ddMMyyyy.
84) How do you lot demo timezone inward formatted appointment inward Java? (answer)
85) The departure betwixt java.util.Date in addition to java.sql.Date inward Java? (answer)
86) How to you lot calculate the departure betwixt 2 dates inward Java? (program)
87) How do you lot convert a String(YYYYMMDD) to appointment inward Java? (answer)
You tin terminate utilisation PowerMock library to examine static methods inward Java.
90) How to do you lot examine a method for an exception using JUnit? (answer)
91) Which unit of measurement testing libraries you lot receive got used for testing Java programs? (answer)
92) What is the departure betwixt @Before in addition to @BeforeClass annotation? (answer)
94) How to write LRU cache inward Java using Generics? (answer)
95) Write a Java computer program to convert bytes to long? (answer)
96) How to contrary a String inward Java without using StringBuffer? (solution)
98) How do you lot depository fiscal establishment check if 2 given String are anagrams? (solution)
99) How to impress all permutation of a String inward Java? (solution)
100) How do you lot impress duplicate elements from an array inward Java? (solution)
101) How to convert String to int inward Java? (solution)
102) How to swap 2 integers without using temp variable? (solution)
103) What is the interface? Why you lot utilisation it if you lot cannot write anything concrete on it?
The interface is used to define API. It tells well-nigh the contract your classes volition follow. It also supports abstraction because a customer tin terminate utilisation interface method to leverage multiple implementations e.g. past times using List interface you lot tin terminate take payoff of random access of ArrayList every bit good every bit flexible insertion in addition to deletion of LinkedList. The interface doesn't allow you lot to write code to maintain things abstract but from Java 8 you lot tin terminate declare static in addition to default methods within interface which are concrete.
104) The departure betwixt abstract degree in addition to interface inward Java? (answer)
There are multiple differences betwixt abstract degree in addition to interface inward Java, but the most of import i is Java's restriction on allowing a degree to extend only i degree but allows it to implement multiple interfaces. An abstract degree is goodness to define default behaviour for a solid unit of measurement of class, but the interface is goodness to define Type which is after used to leverage Polymorphism. Please depository fiscal establishment check the reply for a to a greater extent than thorough give-and-take of this question.
105) Which pattern pattern receive got you lot used inward your production code? apart from Singleton?
This is something you lot tin terminate reply from your experience. You tin terminate to a greater extent than often than non say well-nigh dependency injection, manufactory pattern, decorator pattern or observer pattern, whichever you lot receive got used. Though last prepared to reply follow-up inquiry based upon the pattern you lot choose.
106) Can you lot explicate Liskov Substitution principle? (answer)
This is i of the toughest questions I receive got asked inward Java interviews. Out of 50 candidates, I receive got almost asked alone five receive got managed to reply it. I am non posting an reply to this inquiry every bit I similar you lot to do some research, exercise in addition to pass some fourth dimension to empathize this confusing regulation well.
107) What is Law of Demeter violation? Why it matters? (answer)
Believe it or not, Java is all well-nigh application programming in addition to structuring code. If you receive got goodness cognition of common coding best practices, patterns in addition to what non to do than alone you lot tin terminate write lineament code. Law of Demeter suggests you lot "talk to friends in addition to non stranger", thus used to cut back coupling betwixt classes.
108) What is Adapter pattern? When to utilisation it?
Another oft asked Java pattern pattern questions. It provides interface conversion. If your customer is using some interface but you lot receive got something else, you lot tin terminate write an Adapter to span them together. This is goodness for Java software engineer having 2 to 3 years experience because the inquiry is neither hard nor tricky but requires cognition of OOP pattern patterns.
109) What is "dependency injection" in addition to "inversion of control"? Why would someone utilisation it? (answer)
110) What is an abstract class? How is it unlike from an interface? Why would you lot utilisation it? (answer)
One to a greater extent than classic inquiry from Programming Job interviews, it is every bit quondam every bit chuck Norris. An abstract degree is a degree which tin terminate receive got state, code in addition to implementation, but an interface is a contract which is totally abstract. Since I receive got answered it many times, I am alone giving you lot the gist hither but you lot should read the article linked to reply to larn this useful concept inward much to a greater extent than detail.
111) Which i is improve constructor injection or setter dependency injection? (answer)
Each has their ain payoff in addition to disadvantage. Constructor injection guaranteed that degree volition last initialized with all its dependency, but setter injection provides flexibility to laid an optional dependency. Setter injection is also to a greater extent than readable if you lot are using an XML file to depict dependency. Rule of pollex is to utilisation constructor injection for mandatory dependency in addition to utilisation setter injection for optional dependency.
112) What is departure betwixt dependency injection in addition to manufactory pattern pattern? (answer)
Though both patterns assist to take out object creation constituent from application logic, utilisation of dependency injection results inward cleaner code than manufactory pattern. By using dependency injection, your classes are aught but POJO which alone knows well-nigh dependency but doesn't attention how they are acquired. In the representative of manufactory pattern, the degree also needs to know well-nigh manufactory to acquire dependency. hence, DI results inward to a greater extent than testable classes than manufactory pattern. Please come across the reply for a to a greater extent than detailed give-and-take on this topic.
113) Difference betwixt Adapter in addition to Decorator pattern? (answer)
Though the construction of Adapter in addition to Decorator pattern is similar, the departure comes on the intent of each pattern. The adapter pattern is used to span the gap betwixt 2 interfaces, but Decorator pattern is used to add together novel functionality into the degree without the modifying existing code.
114) Difference betwixt Adapter in addition to Proxy Pattern? (answer)
Similar to the previous question, the departure betwixt Adapter in addition to Proxy patterns is inward their intent. Since both Adapter in addition to Proxy pattern encapsulate the degree which genuinely does the job, thus number inward the same structure, but Adapter pattern is used for interface conversion piece the Proxy pattern is used to add together an extra flat of indirection to back upwardly distribute, controlled or intelligent access.
115) What is Template method pattern? (answer)
Template pattern provides an outline of an algorithm in addition to lets you lot configure or customize its steps. For examples, you lot tin terminate sentiment a sorting algorithm every bit a template to sort object. It defines steps for sorting but allow you lot configure how to compare them using Comparable or something similar inward some other language. The method which outlines the algorithms is also known every bit template method.
116) When do you lot utilisation Visitor pattern pattern? (answer)
The visitor pattern is a solution of occupation where you lot demand to add together functioning on a degree hierarchy but without touching them. This pattern uses double dispatch to add together some other flat of indirection.
117) When do you lot utilisation Composite pattern pattern? (answer)
Composite pattern pattern arranges objects into tree structures to stand upwardly for part-whole hierarchies. It allows clients care for private objects in addition to container of objects uniformly. Use Composite pattern when you lot desire to stand upwardly for part-whole hierarchies of objects.
118) The departure betwixt Inheritance in addition to Composition? (answer)
Though both allows code reuse, Composition is to a greater extent than flexible than Inheritance because it allows you lot to switch to some other implementation at run-time. Code written using Composition is also easier to examine than code involving inheritance hierarchies.
119) Describe overloading in addition to overriding inward Java? (answer)
Both overloading in addition to overriding allow you lot to write 2 methods of unlike functionality but with the same name, but overloading is compile fourth dimension activity piece overriding is run-time activity. Though you lot tin terminate overload a method inward the same class, but you lot tin terminate alone override a method inward tike classes. Inheritance is necessary for overriding.
120) The departure betwixt nested populace static degree in addition to a top flat degree inward Java? (answer)
You tin terminate receive got to a greater extent than than i nested populace static degree within i class, but you lot tin terminate alone receive got i top-level populace degree inward a Java origin file in addition to its call must last same every bit the call of Java origin file.
121) Difference betwixt Composition, Aggregation in addition to Association inward OOP? (answer)
If 2 objects are related to each other, they are said to last associated with each other. Composition in addition to Aggregation are 2 forms of association inward object-oriented programming. The composition is stronger association than Aggregation. In Composition, i object is OWNER of some other object piece inward Aggregation i object is only USER of some other object. If an object Influenza A virus subtype H5N1 is composed of object B in addition to so B doesn't be if Influenza A virus subtype H5N1 ceased to exists, but if object Influenza A virus subtype H5N1 is only an aggregation of object B in addition to so B tin terminate exists fifty-fifty if Influenza A virus subtype H5N1 ceased to exist.
122) Give me an representative of pattern pattern which is based upon opened upwardly closed principle? (answer)
This is i of the practical questions I enquire experienced Java programmer. I await them to know well-nigh OOP pattern principles every bit good every bit patterns. Open closed pattern regulation asserts that your code should last opened upwardly for extension but closed for modification. Which agency if you lot desire to add together novel functionality, you lot tin terminate add together it easily using the novel code but without touching already tried in addition to tested code. There are several pattern patterns which are based upon opened upwardly closed pattern regulation e.g. Strategy pattern if you lot demand a novel strategy, only implement the interface in addition to configure, no demand to modify center logic. One working representative is Collections.sort() method which is based on Strategy pattern in addition to follows the open-closed principle, you lot don't modify sort() method to sort a novel object, what you lot do is only implement Comparator inward your ain way.
123) Difference betwixt Abstract manufactory in addition to Prototype pattern pattern? (answer)
This is the exercise inquiry for you, If you lot are feeling bored only reading in addition to itching to write something, why non write the reply to this question. I would beloved to come across an representative the, which should reply where you lot should utilisation the Abstract manufactory pattern in addition to where is the Prototype pattern is to a greater extent than suitable.
124) When do you lot utilisation Flyweight pattern? (answer)
This is some other pop inquiry from the pattern pattern. Many Java developers with four to half dozen years of experience know the Definition but failed to give whatever concrete example. Since many of you lot powerfulness non receive got used this pattern, it's improve to appear examples from JDK. You are to a greater extent than probable receive got used them earlier in addition to they are piece of cake to call upwardly every bit well. Now let's come across the answer.
Flyweight pattern allows you lot to portion object to back upwardly large numbers without genuinely creating likewise many objects. In club to use Flyweight pattern, you lot demand to brand your object Immutable so that they tin terminate last safely shared. String puddle in addition to puddle of Integer in addition to Long object inward JDK are goodness examples of Flyweight pattern.
125) The departure betwixt nested static degree in addition to top flat class? (answer)
One of the key questions from Java basics. I enquire this inquiry alone to junior Java developers of 1 to 2 years of experience every bit it's likewise piece of cake for an experience Java programmers. The reply is simple, a populace top flat degree must receive got the same call every bit the call of the origin file, at that spot is no such requirement for nested static class. Influenza A virus subtype H5N1 nested degree is e'er within a top flat degree in addition to you lot demand to utilisation the call of the top-level degree to refer nested static degree e.g. HashMap.Entry is a nested static class, where HashMap is a top flat degree in addition to Entry is nested static class.
126) Can you lot write a regular facial expression to depository fiscal establishment check if String is a number? (solution)
If you lot are taking Java interviews in addition to so you lot should enquire at to the lowest degree i inquiry on the regular expression. This clearly differentiates an average programmer with a goodness programmer. Since i of the traits of a goodness developer is to know tools, regex is the best tool for searching something inward the log file, preparing reports etc. Anyway, reply to this inquiry is, a numeric String tin terminate alone incorporate digits i.e. 0 to nine and + in addition to - sign that likewise at start of the String, past times using this information you lot tin terminate write next regular facial expression to depository fiscal establishment check if given String is number or not
127) The departure betwixt checked in addition to unchecked Exception inward Java? (answer)
checked exception is checked past times the compiler at compile time. It's mandatory for a method to either handgrip the checked exception or declare them inward their throws clause. These are the ones which are a sub degree of Exception but doesn't descend from RuntimeException. The unchecked exception is the descendant of RuntimeException in addition to non checked past times the compiler at compile time. This inquiry is at nowadays becoming less pop in addition to you lot would alone let on this with interviews with small-scale companies, both investment banks in addition to startups are moved on from this question.
128) The departure betwixt throw in addition to throws inward Java? (answer)
the throw is used to genuinely throw an instance of java.lang.Throwable class, which agency you lot tin terminate throw both Error in addition to Exception using throw keyword e.g.
On the other hand, throws is used every bit constituent of method annunciation in addition to signals which form of exceptions are thrown past times this method so that its caller tin terminate handgrip them. It's mandatory to declare whatever unhandled checked exception inward throws clause inward Java. Like the previous question, this is some other oft asked Java interview inquiry from errors in addition to exception theme but likewise piece of cake to answer.
129) The departure betwixt Serializable in addition to Externalizable inward Java? (answer)
This is i of the oft asked questions from Java Serialization. The interviewer has been asking this inquiry since the 24-hour interval Serialization was introduced inward Java, but yet alone a few goodness candidate tin terminate reply this inquiry with some confidence in addition to practical knowledge. Serializable interface is used to brand Java classes serializable so that they tin terminate last transferred over network or their Earth tin terminate last saved on disk, but it leverages default serialization built-in JVM, which is expensive, frail in addition to non secure. Externalizable allows you lot to fully command the Serialization process, specify a custom binary format in addition to add together to a greater extent than security measure.
130) The departure betwixt DOM in addition to SAX parser inward Java? (answer)
Another mutual Java inquiry but from XML parsing topic. It's rather unproblematic to reply in addition to that's why many interviewers prefers to enquire this inquiry on the telephonic round. DOM parser loads the whole XML into retention to create a tree based DOM model which helps it rapidly locate nodes in addition to brand a alter inward the construction of XML piece SAX parser is an trial based parser in addition to doesn't charge the whole XML into memory. Due to this argue DOM is faster than SAX but require to a greater extent than retention in addition to non suitable to parse large XML files.
131) Tell me 3 features introduced on JDK 1.7? (answer)
This is i of the goodness questions I enquire to depository fiscal establishment check whether the candidate is aware of recent evolution inward Java technology infinite or not. Even though JDK seven was non a large bang release similar JDK five or JDK 8, it even so has a lot of goodness characteristic to count on e.g. try-with-resource statements, which gratis you lot from closing streams in addition to resources when you lot are done with that, Java automatically closes that. Fork-Join puddle to implement something similar the Map-reduce pattern inward Java. Allowing String variable in addition to literal into switch statements. Diamond operator for improved type inference, no demand to declare generic type on the right-hand side of variable annunciation anymore, results inward to a greater extent than readable in addition to succinct code. Another worth noting characteristic introduced was improved exception treatment e.g. allowing you lot to take take hold of multiple exceptions inward the same take take hold of block.
132) Tell me five features introduced inward JDK 1.8? (answer)
This is the follow-up inquiry of the previous one. Java 8 is path breaking release inward Java's history, hither are the top five features from JDK 8 release
133) What is the departure betwixt Maven in addition to ANT inward Java? (answer)
Another keen inquiry to depository fiscal establishment check the all circular cognition of Java developers. It's piece of cake to reply questions from center Java but when you lot enquire well-nigh setting things up, edifice Java artifacts, many Java software engineer struggles. Coming dorsum to the reply of this question, Though both are build tool in addition to used to create Java application build, Maven is much to a greater extent than than that. It provides criterion construction for Java projection based upon "convention over configuration" concept in addition to automatically deal dependencies (JAR files on which your application is dependent) for Java application. Please come across the reply for to a greater extent than differences betwixt Maven in addition to ANT tool.
That's all guys, lots of Java Interview questions? isn't it? I am certainly if you lot tin terminate reply this listing of Java questions you lot tin terminate easily crevice whatever center Java or advanced Java interview. Though I receive got non included questions from Java EE or J2EE topics e.g. Servlet, JSP, JSF, JPA, JMS, EJB or whatever other Java EE technology or from major spider web frameworks similar Spring MVC, Struts 2.0, Hibernate or both SOAP in addition to RESTful spider web services, it's even so useful for Java developers preparing for Java spider web developer position, because every Java interview starts with questions from fundamentals in addition to JDK API. If you lot think, I receive got missed whatever pop Java inquiry hither in addition to you lot think it should last inward this listing in addition to so experience gratis to propose me. My destination is to create the best listing of Java Interview Questions with latest in addition to greatest inquiry from recent interviews.
Further Learning
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Time is changing in addition to so is Java interviews. Gone are the days, when knowing the departure betwixt String in addition to StringBuffer tin terminate assist you lot to transcend through the 2nd circular of interview, questions are becoming to a greater extent than advanced in addition to interviewers are asking to a greater extent than deep questions. When I started my career, questions similar Vector vs Array and HashMap vs Hashtable were the most pop ones in addition to only memorizing them gives you lot a goodness adventure to do good inward interviews, but non anymore. Nowadays, you lot volition acquire questions from the areas where non many Java programmer looks e.g. NIO, patterns, sophisticated unit of measurement testing or those which are hard to master copy e.g. concurrency, algorithms, information structures in addition to coding.
Since I similar to explore interview questions, I receive got got this huge listing of questions with me, which contains lots in addition to lots of questions on unlike topics. I receive got been preparing this MEGA listing for quite some fourth dimension in addition to at nowadays It's cook to portion with you lot guys.
It contains interview questions non alone from classic topics similar threads, collections, equals in addition to hashcode, sockets but also from NIO, array, string, Java 8 in addition to much more.
It has questions for both entry-level Java programmers in addition to senior developers with years of experience. No affair whether you lot are a Java developer of 1, 2, 3, 4, 5, 6, 8, nine or fifty-fifty 10 years of experience, you lot volition let on something interesting inward this list. It contains questions which are super piece of cake to answer, in addition to also, a inquiry which is tricky plenty for fifty-fifty seasoned Java programmers. If you lot demand to a greater extent than questions in addition to so you lot tin terminate also depository fiscal establishment check out this Java Interview Guide: 200+ Interview Questions with Answers
Given the listing is long in addition to nosotros receive got questions from everywhere, it's imperative that answers must last short, concise in addition to crisp, no fluffing at all. So apart from this paragraph, you lot volition alone listen from me is the questions in addition to answers, no to a greater extent than context, no to a greater extent than feedback in addition to no to a greater extent than evaluation.
For that, I receive got already written spider web log posts, where you lot tin terminate let on my views on a particular question, e.g. why I similar that question, what makes them challenging in addition to what form of reply you lot should await from candidates.
This listing is a footling fleck unlike in addition to I encourage you lot to portion questions in addition to answers inward a similar way so that it should last piece of cake to revise. I promise this listing tin terminate last a keen utilisation for both interviewer in addition to candidates, the interviewer can, of course, position a footling multifariousness on questions to convey novelty in addition to surprise element, which is of import for a goodness interview. While a candidate, tin terminate expand in addition to examine their cognition well-nigh key areas of Java programming linguistic communication in addition to platform.
In 2018 in addition to inward coming years the focus volition last to a greater extent than on advanced concurrency concept, JVM internals, 32-bit vs 64-bit JVM, unit of measurement testing, in addition to create clean code. I am sure, i time you lot read through this MEGA listing of Java interview question, you lot should last able to do good on both telephonic in addition to human face upwardly to human face upwardly programming interviews.
Important Topics for Java Interviews
Apart from quantity, every bit you lot tin terminate come across with a huge number of questions, I receive got worked hard to maintain lineament every bit well. I receive got non alone shared questions from all of import topics but also ensured to include so-called advanced topics which many programmers do non prefer to ready or only left out because they receive got non worked on that.Java NIO in addition to JVM internals questions are best examples of that. You tin terminate maintain pattern patterns also on the same listing but growing number of an experienced programmer are at nowadays good aware of GOF pattern patterns in addition to when to utilisation them. I receive got also worked hard to maintain this listing up-to-date to include what interviewers are asking inward 2018 in addition to what volition last their center focus on coming years. To give you lot an idea, this listing of Java interview questions includes next topics:
- Multithreading, concurrency in addition to thread basics
- Date type conversion in addition to fundamentals
- Garbage Collection
- Java Collections Framework
- Array
- String
- GOF Design Patterns
- SOLID pattern principles
- Abstract degree in addition to interface
- Java basics e.g. equals() in addition to hashcode
- Generics in addition to Enum
- Java IO in addition to NIO
- Common Networking protocols
- Data construction in addition to algorithm inward Java
- Regular expressions
- JVM internals
- Java Best Practices
- JDBC
- Date, Time, in addition to Calendar
- XML Processing inward Java
- JUnit
- Programming
You guys are also lucky that nowadays at that spot are some goodness books available to ready for Java interviews, i of them which I especially let on useful in addition to interesting to read is Java Programming Interview Exposed past times Markham. It volition take you lot to some of the most of import topics for Java in addition to JEE interviews, worth reading fifty-fifty if you lot are non preparing for Java interview.
Top 120 Java Interview Questions Answers
So at nowadays the fourth dimension has come upwardly to innovate you lot to this MEGA listing of 120 Java questions collected from diverse interviews of in conclusion five years. I am certainly you lot receive got seen many of these questions personally on your interviews in addition to many of you lot would receive got answered them correctly every bit well.Multithreading, Concurrency in addition to Thread basics Questions
1) Can nosotros brand array volatile inward Java?
This is i of the tricky Java multi-threading questions you lot volition come across inward senior Java developer Interview. Yes, you lot tin terminate brand an array volatile inward Java but alone the reference which is pointing to an array, non the whole array. What I mean, if i thread changes the reference variable to points to some other array, that volition render a volatile guarantee, but if multiple threads are changing private array elements they won't last having happens earlier guarantee provided past times the volatile modifier.
2) Can volatile brand a non-atomic functioning to atomic?
This some other goodness inquiry I beloved to enquire on volatile, mostly every bit a follow-up of the previous question. This inquiry is also non piece of cake to reply because volatile is non well-nigh atomicity, but at that spot are cases where you lot tin terminate utilisation a volatile variable to brand the functioning atomic.
One representative I receive got seen is having a long acre inward your class. If you lot know that a long acre is accessed past times to a greater extent than than i thread e.g. a counter, a cost acre or anything, you lot improve transcend far volatile. Why? because reading to a long variable is non atomic inward Java in addition to done inward 2 steps, If i thread is writing or updating long value, it's possible for some other thread to come across one-half value (fist 32-bit). While reading/writing a volatile long or double (64 bit) is atomic.
3) What are practical uses of volatile modifier?
One of the practical utilisation of the volatile variable is to brand reading double in addition to long atomic. Both double in addition to long are 64-bit broad in addition to they are read inward 2 parts, get-go 32-bit get-go fourth dimension in addition to side past times side 32-bit 2nd time, which is non-atomic but volatile double in addition to long read is atomic inward Java. Another utilisation of the volatile variable is to render a retention barrier, only similar it is used inward Disrupter framework. Basically, Java Memory model inserts a write barrier after you lot write to a volatile variable in addition to a read barrier earlier you lot read it. Which means, if you lot write to volatile acre in addition to so it's guaranteed that whatever thread accessing that variable volition come across the value you lot wrote in addition to anything you lot did earlier doing that right into the thread is guaranteed to receive got happened in addition to whatever updated information values volition also last visible to all threads, because the retention barrier flushed all other writes to the cache.
4) What guarantee volatile variable provides? (answer)
volatile variables render the guarantee well-nigh ordering in addition to visibility e.g. volatile assignment cannot last re-ordered with other statements but inward the absence of whatever synchronization didactics compiler, JVM or JIT are gratis to reorder statements for improve performance. volatile also provides the happens-before guarantee which ensures changes made inward i thread is visible to others. In some cases volatile also render atomicity e.g. reading 64-bit information types similar long in addition to double are non atomic but read of volatile double or long is atomic.
5) Which i would last piece of cake to write? synchronization code for 10 threads or 2 threads?
In price of writing code, both volition last of same complexity because synchronization code is independent of a number of threads. Choice of synchronization though depends upon a number of threads because the number of thread nowadays to a greater extent than contention, so you lot transcend for advanced synchronization technique e.g. lock stripping, which requires to a greater extent than complex code in addition to expertise.
6) How do you lot telephone outcry upwardly wait() method? using if block or loop? Why? (answer)
wait() method should e'er last called inward loop because it's possible that until thread gets CPU to start running over again the status powerfulness non hold, so it's e'er improve to depository fiscal establishment check status inward loop earlier proceeding. Here is the criterion idiom of using hold off in addition to notify method inward Java:
// The criterion idiom for using the hold off method synchronized (obj) { while (condition does non hold) obj.wait(); // (Releases lock, in addition to reacquires on wakeup) ... // Perform activity appropriate to condition }See Effective Java Item 69 to larn to a greater extent than well-nigh why hold off method should telephone outcry upwardly inward the loop.
7) What is imitation sharing inward the context of multi-threading?
imitation sharing is i of the well-known performance issues on multi-core systems, where each procedure has its local cache. imitation sharing occurs when threads on unlike processor modify variables that reside on same cache delineate every bit shown inward the next image:
False sharing is rattling hard to let on because the thread may last accessing completely unlike global variables that occur to last relatively unopen together inward memory. Like many concurrency issues, the primary way to avoid imitation sharing is careful code review in addition to aligning your information construction with the size of a cache line.
8) What is busy spin? Why should you lot utilisation it?
Busy spin is i of the technique to hold off for events without releasing CPU. It's often done to avoid losing information inward CPU cached which is lost if the thread is paused in addition to resumed inward some other core. So, if you lot are working on depression latency organisation where your club processing thread currently doesn't receive got whatever order, instead of sleeping or calling wait(), you lot tin terminate only loop in addition to and so over again depository fiscal establishment check the queue for novel messages. It's alone beneficial if you lot demand to hold off for a rattling small-scale amount of fourth dimension e.g. inward micro seconds or nano seconds. LMAX Disrupter framework, a high-performance inter-thread messaging library has a BusySpinWaitStrategy which is based on this concept in addition to uses a busy spin loop for EventProcessors waiting on the barrier.
9) How do you lot take thread dump inward Java?
You tin terminate take a thread dump of Java application inward Linux past times using kill -3 PID, where PID is the procedure id of Java process. In Windows, you lot tin terminate press Ctrl + Break. This volition instruct JVM to impress thread dump inward criterion out or err in addition to it could transcend to console or log file depending upon your application configuration. If you lot receive got used Tomcat in addition to so when
10) is Swing thread-safe? (answer)
No, Swing is non thread-safe. You cannot update Swing components e.g. JTable, JList or JPanel from whatever thread, inward fact, they must last updated from GUI or AWT thread. That's why swings render invokeAndWait() in addition to invokeLater() method to asking GUI update from whatever other threads. This methods position update asking inward AWT threads queue in addition to tin terminate hold off till update or render straightaway for an asynchronous update. You tin terminate also depository fiscal establishment check the detailed reply to larn more.
11) What is a thread local variable inward Java? (answer)
Thread-local variables are variables confined to a thread, its similar thread's ain re-create which is non shared betwixt multiple threads. Java provides a ThreadLocal degree to back upwardly thread-local variables. It's i of the many ways to accomplish thread-safety. Though last careful piece using thread local variable inward manged environs e.g. with spider web servers where worker thread out lives whatever application variable. Any thread local variable which is non removed i time its piece of work is done tin terminate potentially displace a retention leak inward Java application.
12) Write wait-notify code for producer-consumer problem? (answer)
Please come across the reply for a code example. Just call upwardly to telephone outcry upwardly wait() in addition to notify() method from synchronized block in addition to examine waiting for status on the loop instead of if block.
13) Write code for thread-safe Singleton inward Java? (answer)
Please come across the reply for a code representative in addition to stair past times stair guide to creating thread-safe singleton degree inward Java. When nosotros say thread-safe, which agency Singleton should rest singleton fifty-fifty if initialization occurs inward the representative of multiple threads. Using Java enum every bit Singleton degree is i of the easiest ways to create a thread-safe singleton inward Java.
14) The departure betwixt slumber in addition to hold off inward Java? (answer)
Though both are used to interruption currently running thread, sleep() is genuinely meant for brusk interruption because it doesn't release lock, piece wait() is meant for conditional hold off in addition to that's why it release lock which tin terminate in addition to so last acquired past times some other thread to alter the status on which it is waiting.
15) What is an immutable object? How do you lot create an Immutable object inward Java? (answer)
Immutable objects are those whose Earth cannot last changed i time created. Any modification volition number inward a novel object e.g. String, Integer, and other wrapper class. Please come across the reply for stair past times stair guide to creating Immutable degree inward Java.
16) Can nosotros create an Immutable object, which contains a mutable object?
Yes, its possible to create an Immutable object which may incorporate a mutable object, you lot only demand to last a footling fleck careful non to portion the reference of the mutable component, instead, you lot should render a re-create of it if you lot receive got to. Most mutual representative is an Object which incorporate the reference of java.util.Date object.
Date types in addition to Basic Java Interview Questions
17) What is the right information type to stand upwardly for a cost inward Java? (answer)
BigDecimal if retention is non a delineate of piece of work organisation in addition to Performance is non critical, otherwise double with predefined precision.
18) How do you lot convert bytes to String? (answer)
you lot tin terminate convert bytes to the string using string constructor which accepts byte[], only brand certainly that right grapheme encoding otherwise platform's default grapheme encoding volition last used which may or may non last same.
19) How do you lot convert bytes to long inward Java? (answer)
This questions if for you lot to reply :-)
20) Can nosotros cast an int value into byte variable? what volition occur if the value of int is larger than byte?
Yes, nosotros tin terminate cast but int is 32 fleck long inward coffee piece byte is 8 fleck long inward coffee so when you lot cast an int to byte higher 24 bits are lost in addition to a byte tin terminate alone concur a value from -128 to 128.
21) There are 2 classes B extends Influenza A virus subtype H5N1 in addition to C extends B, Can nosotros cast B into C e.g. C = (C) B; (answer)
22) Which degree contains clone method? Cloneable or Object? (answer)
java.lang.Cloneable is marking interface in addition to doesn't incorporate whatever method clone method is defined inward the object class. It is also knowing that clone() is a native method agency it's implemented inward C or C++ or whatever other native language.
23) Is ++ operator is thread-safe inward Java? (answer)
No it's non a thread rubber operator because its involve multiple instructions similar reading a value, incriminating it in addition to storing it dorsum into retention which tin terminate last overlapped betwixt multiple threads.
24) Difference betwixt a = a + b in addition to a += b ? (answer)
The += operator implicitly cast the number of add-on into the type of variable used to concur the result. When you lot add together 2 integral variable e.g. variable of type byte, short, or int in addition to so they are get-go promoted to int in addition to them add-on happens. If number of add-on is to a greater extent than than maximum value of a in addition to so a + b volition give compile fourth dimension fault but a += b volition last ok every bit shown below
byte a = 127; byte b = 127; b = a + b; // fault : cannot convert from int to byte b += a; // ok
25) Can I shop a double value inward a long variable without casting? (answer)
No, you lot cannot shop a double value into a long variable without casting because the attain of double is to a greater extent than that long in addition to you lot nosotros demand to type cast. It's non dificult to reply this inquiry but many develoepr acquire it wrong due to confusion on which i is bigger betwixt double in addition to long inward Java.
26) What volition this render 3*0.1 == 0.3? truthful or false? (answer)
This is i of the genuinely tricky questions. Out of 100, alone five developers answered this inquiry in addition to alone of them receive got explained the concept correctly. The brusk reply is imitation because some floating dot numbers tin terminate non last represented exactly.
27) Which i volition take to a greater extent than memory, an int or Integer? (answer)
An Integer object volition take to a greater extent than retention an Integer is the an object in addition to it store meta information overhead well-nigh the object in addition to int is primitive type so its takes less space.
28) Why is String Immutable inward Java? (answer)
One of my favorite Java interview question. The String is Immutable inward coffee because coffee designer thought that string volition last heavily used in addition to making it immutable allow some optimization piece of cake sharing same String object betwixt multiple clients. See the link for the to a greater extent than detailed answer. This is a keen inquiry for Java programmers with less experience every bit it gives them nutrient for thought, to think well-nigh how things industrial plant inward Java, what Jave designers powerfulness receive got thought when they created String degree etc.
29) Can nosotros utilisation String inward the switch case? (answer)
Yes from Java seven onward nosotros tin terminate utilisation String inward switch representative but it is only syntactic sugar. Internally string hash code is used for the switch. See the detaiedl reply for to a greater extent than explanation in addition to discussion.
30) What is constructor chaining inward Java? (answer)
When you lot telephone outcry upwardly i constructor from other than it's known every bit constructor chaining inward Java. This happens when you lot receive got multiple, overloaded constructor inward the class.
JVM Internals in addition to Garbage Collection Interview Questions
In the twelvemonth 2018 I receive got seen increased focus on JVM internal in addition to Garbage collection tuning, monitoring Java appliation, dealing with Java performance issues on diverse Java interviews. This is genuinely transcend the prime number theme for interviewing whatever experienced Java developer for senior seat e.g. technical lead, VP or squad lead. If you lot experience you lot are brusk of experience in addition to cognition inward this expanse in addition to so you lot should read atleast i mass mentioned inward my listing of Java Performance books. I vote goes to Java Performance, The Definitive guide past times Scott.31) What is the size of int inward 64-bit JVM?
The size of an int variable is constant inward Java, it's e'er 32-bit irrespective of platform. Which agency the size of primitive int is same inward both 32-bit in addition to 64-bit Java virtual machine.
32) The departure betwixt Serial in addition to Parallel Garbage Collector? (answer)
Even though both the series in addition to parallel collectors displace a stop-the-world interruption during Garbage collection. The chief departure betwixt them is that a series collector is a default copying collector which uses alone i GC thread for garbage collection piece a parallel collector uses multiple GC threads for garbage collection.
33) What is the size of an int variable inward 32-bit in addition to 64-bit JVM? (answer)
The size of int is same inward both 32-bit in addition to 64-bit JVM, it's e'er 32 bits or four bytes.
34) Influenza A virus subtype H5N1 departure betwixt WeakReference in addition to SoftReference inward Java? (answer)
Though both WeakReference in addition to SoftReference helps garbage collector in addition to retention efficient, WeakReference becomes eligible for garbage collection every bit presently every bit in conclusion strong reference is lost but SoftReference fifty-fifty thought it tin terminate non preclude GC, it tin terminate delay it until JVM absolutely demand memory.
35) How do WeakHashMap works? (answer)
WeakHashMap industrial plant similar a normal HashMap but uses WeakReference for keys, which agency if the key object doesn't receive got whatever reference in addition to so both key/value mapping volition transcend eligible for garbage collection.
36) What is -XX:+UseCompressedOops JVM option? Why utilisation it? (answer)
When you lot transcend migrate your Java application from 32-bit to 64-bit JVM, the heap requirement of a abrupt increases, almost double, due to increasing size of ordinary object pointer from 32 fleck to 64 bit. This also adversely impact how much information you lot tin terminate maintain inward CPU cache, which is much smaller than memory. Since chief motivation for moving to 64-bit JVM is to specify large heap size, you lot tin terminate salve some retention past times using compressed OOP. By using -XX:+UseCompressedOops, JVM uses 32-bit OOP instead of 64-bit OOP.
37) How do you lot let on if JVM is 32-bit or 64-bit from Java Program? (answer)
You tin terminate let on that past times checking some organisation properties like sun.arch.data.model or os.arch
38) What is the maximum heap size of 32-bit in addition to 64-bit JVM? (answer)
Theoretically, the maximum heap retention you lot tin terminate assign to a 32-bit JVM is 2^32 which is 4GB but practically the bound is much smaller. It also varies betwixt operating systems e.g. shape 1.5GB inward Windows to almost 3GB inward Solaris. 64-bit JVM allows you lot to specify larger heap size, theoretically 2^64 which is quite large but practically you lot tin terminate specify heap infinite upwardly to 100GBs. There are fifty-fifty JVM e.g. Azul where heap infinite of M gigs is also possible.
39) What is the departure betwixt JRE, JDK, JVM in addition to JIT? (answer)
JRE stands for Java run-time in addition to it's required to run Java application. JDK stands for Java evolution kit in addition to provides tools to railroad train Java computer program e.g. Java compiler. It also contains JRE. The JVM stands for Java virtual machine in addition to it's the procedure responsible for running Java application. The JIT stands for Just In Time compilation in addition to helps to boost the performance of Java application past times converting Java byte code into native code when the crossed certainly threshold i.e. mainly hot code is converted into native code.
40) Explain Java Heap infinite in addition to Garbage collection? (answer)
When a Java procedure is started using coffee command, retention is allocated to it. Part of this retention is used to create heap space, which is used to allocate retention to objects whenever they are created inward the program. Garbage collection is the procedure within JVM which reclaims retention from dead objects for time to come allocation.
41) Can you lot guarantee the garbage collection process? (answer)
No, you lot cannot guarantee the garbage collection, though you lot tin terminate brand a asking using System.gc() or Runtime.gc() method.
42) How do you lot let on retention usage from Java program? How much percentage of the heap is used?
You tin terminate utilisation retention related methods from java.lang.Runtime degree to acquire the gratis memory, total retention in addition to maximum heap retention inward Java. By using these methods, you lot tin terminate let on out how many percents of the heap is used in addition to how much heap infinite is remaining. Runtime.freeMemory() render amount of gratis retention inward bytes, Runtime.totalMemory() returns total retention inward bytes in addition to Runtime.maxMemory() returns maximum retention inward bytes.
43) What is the departure betwixt stack in addition to heap inward Java? (answer)
Stack in addition to heap are unlike retention areas inward the JVM in addition to they are used for unlike purposes. The stack is used to concur method frames in addition to local variables piece objects are e'er allocated retention from the heap. The stack is commonly much smaller than heap retention in addition to also didn't shared betwixt multiple threads, but heap is shared with all threads inward JVM.
Basic Java concepts Interview Questions
44) What's the departure betwixt "a == b" in addition to "a.equals(b)"? (answer)The a = b does object reference matching if both a in addition to b are an object in addition to alone render truthful if both are pointing to the same object inward the heap space, on the other hand, a.equals(b) is used for logical mapping in addition to its expected from an object to override this method to render logical equality. For example, String degree overrides this equals() method so that you lot tin terminate compare 2 Strings, which are the unlike object but contains same letters.
45) What is a.hashCode() used for? How is it related to a.equals(b)? (answer)
hashCode() method returns an int hash value corresponding to an object. It's used inward hash based collection classes e.g Hashtable, HashMap, LinkedHashMap in addition to so on. It's rattling tightly related to equals() method. According to Java specification, 2 objects which are equal to each other using equals() method must receive got same hash code.
46) Difference betwixt final, finalize in addition to finally? (answer)
The in conclusion is a modifier which you lot tin terminate apply to variable, methods in addition to classes. If you lot brand a variable in conclusion it agency its value cannot last changed i time initialized. finalize is a method, which is called only earlier an object is a garbage collected, giving it in conclusion adventure to resurrect itself, but the telephone outcry upwardly to finalize is non guaranteed. finally is a keyword which is used inward exception treatment along with endeavour in addition to catch. the finally block is e'er executed irrespective of whether an exception is thrown from endeavour block or not.
47) What is a compile fourth dimension constant inward Java? What is the endangerment of using it?
populace static in conclusion variables are also known every bit a compile fourth dimension constant, the populace is optional there. They are replaced with actual values at compile fourth dimension because compiler know their value up-front in addition to also knows that it cannot last changed during run-time. One of the occupation with this is that if you lot happened to utilisation a populace static in conclusion variable from some in-house or 3rd political party library in addition to their value changed after than your customer volition even so last using quondam value fifty-fifty after you lot deploy a novel version of JARs. To avoid that, brand certainly you lot compile your computer program when you lot upgrade dependency JAR files.
Java Collections Framework Interview Questions
It also contains Data construction in addition to algorithm Interview inquiry inward Java, questions on array, linked list, HashMap, ArrayList, Hashtable, Stack, Queue, PriorityQueue, LinkedHashMap in addition to ConcurrentHashMap.48) The departure betwixt List, Set, Map, in addition to Queue inward Java? (answer)
The listing is an ordered collection which allows duplicate. It also has an implementation which provides constant fourth dimension index based access, but that is non guaranteed past times List interface. Set is unordered collection which
49) Difference betwixt poll() in addition to remove() method?
Both poll() in addition to remove() take out the object from the Queue but if poll() fails in addition to so it returns zero but if take away fails it throws Exception.
50) The departure betwixt LinkedHashMap in addition to PriorityQueue inward Java? (answer)
PriorityQueue guarantees that lowest or highest priority chemical element e'er rest at the caput of the queue, but LinkedHashMap maintains the club on which elements are inserted. When you lot iterate over a PriorityQueue, iterator doesn't guarantee whatever club but iterator of LinkedHashMap does guarantee the club on which elements are inserted.
51) Difference betwixt ArrayList in addition to LinkedList inward Java? (answer)
The obvious departure betwixt them is that ArrrayList is backed past times array information structure, supprots random access in addition to LinkedList is backed past times linked listing information construction in addition to doesn't supprot random access. Accessing an chemical element with the index is O(1) inward ArrayList but its O(n) inward LinkedList. See the reply for to a greater extent than detailed discussion.
52) What is a pair of ways that you lot could sort a collection? (answer)
You tin terminate either utilisation the Sorted collection similar TreeSet or TreeMap or you lot tin terminate sort using the ordered collection similar a listing in addition to using Collections.sort() method.
53) How do you lot impress Array inward Java? (answer)
You tin terminate impress an array past times using the Arrays.toString() in addition to Arrays.deepToString() method. Since array doesn't implement toString() past times itself, only passing an array to System.out.println() volition non impress its contents but Arrays.toString() volition impress each element.
54) LinkedList inward Java is doubly or singly linked list? (answer)
It's a doubly linked list, you lot tin terminate depository fiscal establishment check the code inward JDK. In Eclipse, you lot tin terminate utilisation the shortcut, Ctrl + T to straight opened upwardly this degree inward Editor.
55) Which form of tree is used to implement TreeMap inward Java? (answer)
Influenza A virus subtype H5N1 Red Black tree is used to implement TreeMap inward Java.
56) What is the departure betwixt Hashtable in addition to HashMap? (answer)It's a doubly linked list, you lot tin terminate depository fiscal establishment check the code inward JDK. In Eclipse, you lot tin terminate utilisation the shortcut, Ctrl + T to straight opened upwardly this degree inward Editor.
55) Which form of tree is used to implement TreeMap inward Java? (answer)
Influenza A virus subtype H5N1 Red Black tree is used to implement TreeMap inward Java.
There are many differences betwixt these 2 classes, some of them are following:
a) Hashtable is a legacy degree in addition to nowadays from JDK 1, HashMap was added later.
b) Hashtable is synchronized in addition to slower but HashMap is non synchronized in addition to faster.
c) Hashtable doesn't allow zero keys but HashMap allows i zero key.
See the reply for to a greater extent than differences betwixt HashMap in addition to Hashtable inward Java.
57) How HashSet industrial plant internally inward Java? (answer)
HashSet is internally implemented using an HashMap. Since a Map needs key in addition to value, a default value is used for all keys. Similar to HashMap, HashSet doesn't allow duplicate keys in addition to alone i zero key, I hateful you lot tin terminate alone shop i zero object inward HashSet.
58) Write code to take away elements from ArrayList piece iterating? (answer)
Key hither is to depository fiscal establishment check whether candidate uses ArrayList's remove() or Iterator's remove(). Here is the sample code which uses right way o take away elements from ArrayList piece looping over in addition to avoids ConcurrentModificationException.
59) Can I write my ain container degree in addition to utilisation it inward the for-each loop?
Yes, you lot tin terminate write your ain container class. You demand to implement the Iterable interface if you lot desire to loop over advanced for loop inward Java, though. If you lot implement Collection in addition to so you lot past times default acquire that property.
60) What is default size of ArrayList in addition to HashMap inward Java? (answer)
As of Java seven now, default size of ArrayList is 10 in addition to default capacity of HashMap is 16, it must last powerfulness of 2. Here is code snippet from ArrayList and HashMap degree :
// from ArrayList.java JDK 1.7 private static final int DEFAULT_CAPACITY = 10; //from HashMap.java JDK 7 static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
61) Is it possible for 2 unequal objects to receive got the same hashcode?
Yes, 2 unequal objects tin terminate receive got same hashcode that's why collision occur inward a hashmap.
the equal hashcode contract alone says that 2 equal objects must receive got the same hashcode it doesn't say anything well-nigh the unequal object.
62) Can 2 equal object receive got the unlike hash code?
No, thats non possible according to hash code contract.
63) Can nosotros utilisation random numbers inward the hashcode() method? (answer)
No, because hashcode of an object should last e'er same. See the reply to learning to a greater extent than well-nigh things to call upwardly piece overriding hashCode() method inward Java.
64) What is the departure betwixt Comparator in addition to Comparable inward Java? (answer)
The Comparable interface is used to define the natural club of object piece Comparator is used to define custom order. Comparable tin terminate last e'er one, but nosotros tin terminate receive got multiple comparators to define customized club for objects.
65) Why you lot demand to override hashcode, when you lot override equals inward Java? (answer)
Because equals receive got code contract mandates to override equals in addition to hashcode together .since many container degree similar HashMap or HashSet depends on hashcode in addition to equals contract.
Java IO in addition to NIO Interview questions
IO is rattling of import from Java interview dot of view. You should receive got a goodness cognition of quondam Java IO, NIO, in addition to NIO2 alsong with some operating organisation in addition to disk IO fundamentals. Here are some oft asked questions shape Java IO.66) In my Java program, I receive got iii sockets? How many threads I volition demand to handgrip that?
67) How do you lot create ByteBuffer inward Java?
68) How do you lot write in addition to read from ByteBuffer inward Java?
69) Is Java BIG endian or LITTLE endian?
70) What is the byte club of ByteBuffer?
71) The departure betwixt direct buffer in addition to non-direct buffer inward Java? (answer)
72) What is the retention mapped buffer inward Java? (answer)
73) What is TCP NO DELAY socket option?
74) What is the departure betwixt TCP in addition to UDP protocol? (answer)
75) The departure betwixt ByteBuffer in addition to StringBuffer inward Java? (answer)
Java Best Practices Interview question
Contains best practices from unlike parts of Java programming e.g. Collections, String, IO, Multi-threading, Error in addition to Exception handling, pattern patterns etc. This department is mostly for experience Java developer, technical lead, AVP, squad Pb in addition to coders who are responsible for products. If you lot desire to create lineament products you lot must know in addition to follow the best practices.76) What best practices you lot follow piece writing multi-threaded code inward Java? (answer)
Here are pair of best practices which I follow piece writing concurrent code inward Java:
a) Always call your thread, this volition assist inward debugging.
b) minimize the reach of synchronization, instead of making whole method synchronized, alone critical department should last synchronized.
c) prefer volatile over synchronized if you lot tin terminate can.
e) utilisation higher flat concurrency utilities instead of waitn() in addition to notify for inter thread communication e.g. BlockingQueue, CountDownLatch in addition to Semeaphore.
e) Prefer concurrent collection over synchronized collection inward Java. They render improve scalability.
77) Tell me few best practices you lot apply piece using Collections inward Java? (answer)
Here are pair of best practices I follow piece using Collectionc classes from Java:
a) Always utilisation the right collection e.g. if you lot demand non-synchronized listing in addition to so utilisation ArrayList in addition to non Vector.
b) Prefer concurrent collection over synchronized collection because they are to a greater extent than scalable.
c) Always utilisation interface to a stand upwardly for in addition to access a collection e.g. utilisation List to shop ArrayList, Map to shop HashMap in addition to so on.
d) Use iterator to loop over collection.
e) Always utilisation generics with collection.
78) Can you lot tell us at to the lowest degree five best exercise you lot utilisation piece using threads inward Java? (answer)
This is similar to the previous inquiry in addition to you lot tin terminate utilisation the reply given there. Particularly with thread, you lot should:
a) call your thread
b) maintain your task in addition to thread separate, utilisation Runnable or Callable with thread puddle executor.
c) utilisation thread pool
d) utilisation volatile to call for compiler well-nigh ordering, visibility, in addition to atomicity.
e) avoid thread local variable because wrong utilisation of ThreadLocal degree inward Java tin terminate create a retention leak.
Look at that spot are many best practices in addition to I give extra points to the developer which convey something new, something fifty-fifty I don't know. I brand certainly to enquire this inquiry to Java developers of 8 to 10 years of experience only to guess his hands on experience in addition to knowledge.
79) Name five IO best practices? (answer)
IO is rattling of import for performance of your Java application. Ideally you lot should avoid IO inward critical path of your application. Here are pair of Java IO best practices you lot tin terminate follow:
a) Use buffered IO classes instead of reading private bytes in addition to char.
b) Use classes from NIO in addition to NIO2
c) Always unopen streams inward finally block or utilisation try-with-resource statements.
d) utilisation retention mapped file for faster IO.
If a Java candidate doesn't know well-nigh IO in addition to NIO, especially if he has at least 2 to four years of experience, he needs some reading.80) Name five JDBC best practices your follow? (answer)
Another goodness Java best practices for experienced Java developer of seven to 8 years experience. Why it's important? because they are the ones which laid the tendency inward the code in addition to educate junior developers. There are many best practices in addition to you lot tin terminate call every bit per your confort in addition to conviniece. Here are some of the to a greater extent than mutual ones:
a) utilisation batch declaration for inserting in addition to updating data.
b) utilisation PreparedStatement to avoid SQL exception in addition to improve performance.
c) utilisation database connective pool
d) access resultset using column call instead of column indexes.
e) Don't generate dynamic SQL past times concatenating String with user input.
81) Name pair of method overloading best practices inward Java? (answer)
Here are some best practices you lot tin terminate follow piece overloading a method inward Java to avoid confusion with auto-boxing:
a) Don't overload method where i accepts int in addition to other accepts Integer.
b) Don't overload method where number of declaration is same in addition to alone club of declaration is different.
c) Use varargs after overloaded methods has to a greater extent than than five arguments.
Date, Time in addition to Calendar Interview questions inward Java
82) Does SimpleDateFormat is rubber to utilisation inward the multi-threaded program? (answer)
No, unfortunately, DateFormat in addition to all its implementations including SimpleDateFormat is non thread-safe, thus should non last used inward the multi-threaded computer program until external thread-safety measures are applied e.g. confining SimpleDateFormat object into a ThreadLocal variable. If you lot don't do that, you lot volition acquire an wrong number piece parsing or formatting dates inward Java. Though, for all practical appointment fourth dimension purpose, I highly recommend joda-time library.
83) How do you lot format a appointment inward Java? e.g. inward the ddMMyyyy format? (answer)
You tin terminate either utilisation SimpleDateFormat degree or joda-time library to format appointment inward Java. DateFormat degree allows you lot to format appointment on many pop formats. Please come across the reply for code samples to format appointment into unlike formats e.g. dd-MM-yyyy or ddMMyyyy.
84) How do you lot demo timezone inward formatted appointment inward Java? (answer)
85) The departure betwixt java.util.Date in addition to java.sql.Date inward Java? (answer)
86) How to you lot calculate the departure betwixt 2 dates inward Java? (program)
87) How do you lot convert a String(YYYYMMDD) to appointment inward Java? (answer)
Unit testing JUnit Interview questions
89) How do you lot examine static method? (answer)You tin terminate utilisation PowerMock library to examine static methods inward Java.
90) How to do you lot examine a method for an exception using JUnit? (answer)
91) Which unit of measurement testing libraries you lot receive got used for testing Java programs? (answer)
92) What is the departure betwixt @Before in addition to @BeforeClass annotation? (answer)
Programming in addition to Coding Questions
93) How to depository fiscal establishment check if a String contains alone numeric digits? (solution)94) How to write LRU cache inward Java using Generics? (answer)
95) Write a Java computer program to convert bytes to long? (answer)
96) How to contrary a String inward Java without using StringBuffer? (solution)
98) How do you lot depository fiscal establishment check if 2 given String are anagrams? (solution)
99) How to impress all permutation of a String inward Java? (solution)
100) How do you lot impress duplicate elements from an array inward Java? (solution)
101) How to convert String to int inward Java? (solution)
102) How to swap 2 integers without using temp variable? (solution)
Java Interview questions from OOP in addition to Design Patterns
It contains Java Interview questions from SOLID pattern principles, OOP fundamentals e.g. class, object, interface, Inheritance, Polymorphism, Encapsulation, in addition to Abstraction every bit good every bit to a greater extent than advanced concepts similar Composition, Aggregation, in addition to Association. It also contains questions from GOF pattern patterns.103) What is the interface? Why you lot utilisation it if you lot cannot write anything concrete on it?
The interface is used to define API. It tells well-nigh the contract your classes volition follow. It also supports abstraction because a customer tin terminate utilisation interface method to leverage multiple implementations e.g. past times using List interface you lot tin terminate take payoff of random access of ArrayList every bit good every bit flexible insertion in addition to deletion of LinkedList. The interface doesn't allow you lot to write code to maintain things abstract but from Java 8 you lot tin terminate declare static in addition to default methods within interface which are concrete.
104) The departure betwixt abstract degree in addition to interface inward Java? (answer)
There are multiple differences betwixt abstract degree in addition to interface inward Java, but the most of import i is Java's restriction on allowing a degree to extend only i degree but allows it to implement multiple interfaces. An abstract degree is goodness to define default behaviour for a solid unit of measurement of class, but the interface is goodness to define Type which is after used to leverage Polymorphism. Please depository fiscal establishment check the reply for a to a greater extent than thorough give-and-take of this question.
105) Which pattern pattern receive got you lot used inward your production code? apart from Singleton?
This is something you lot tin terminate reply from your experience. You tin terminate to a greater extent than often than non say well-nigh dependency injection, manufactory pattern, decorator pattern or observer pattern, whichever you lot receive got used. Though last prepared to reply follow-up inquiry based upon the pattern you lot choose.
106) Can you lot explicate Liskov Substitution principle? (answer)
This is i of the toughest questions I receive got asked inward Java interviews. Out of 50 candidates, I receive got almost asked alone five receive got managed to reply it. I am non posting an reply to this inquiry every bit I similar you lot to do some research, exercise in addition to pass some fourth dimension to empathize this confusing regulation well.
107) What is Law of Demeter violation? Why it matters? (answer)
Believe it or not, Java is all well-nigh application programming in addition to structuring code. If you receive got goodness cognition of common coding best practices, patterns in addition to what non to do than alone you lot tin terminate write lineament code. Law of Demeter suggests you lot "talk to friends in addition to non stranger", thus used to cut back coupling betwixt classes.
108) What is Adapter pattern? When to utilisation it?
Another oft asked Java pattern pattern questions. It provides interface conversion. If your customer is using some interface but you lot receive got something else, you lot tin terminate write an Adapter to span them together. This is goodness for Java software engineer having 2 to 3 years experience because the inquiry is neither hard nor tricky but requires cognition of OOP pattern patterns.
109) What is "dependency injection" in addition to "inversion of control"? Why would someone utilisation it? (answer)
110) What is an abstract class? How is it unlike from an interface? Why would you lot utilisation it? (answer)
One to a greater extent than classic inquiry from Programming Job interviews, it is every bit quondam every bit chuck Norris. An abstract degree is a degree which tin terminate receive got state, code in addition to implementation, but an interface is a contract which is totally abstract. Since I receive got answered it many times, I am alone giving you lot the gist hither but you lot should read the article linked to reply to larn this useful concept inward much to a greater extent than detail.
111) Which i is improve constructor injection or setter dependency injection? (answer)
Each has their ain payoff in addition to disadvantage. Constructor injection guaranteed that degree volition last initialized with all its dependency, but setter injection provides flexibility to laid an optional dependency. Setter injection is also to a greater extent than readable if you lot are using an XML file to depict dependency. Rule of pollex is to utilisation constructor injection for mandatory dependency in addition to utilisation setter injection for optional dependency.
112) What is departure betwixt dependency injection in addition to manufactory pattern pattern? (answer)
Though both patterns assist to take out object creation constituent from application logic, utilisation of dependency injection results inward cleaner code than manufactory pattern. By using dependency injection, your classes are aught but POJO which alone knows well-nigh dependency but doesn't attention how they are acquired. In the representative of manufactory pattern, the degree also needs to know well-nigh manufactory to acquire dependency. hence, DI results inward to a greater extent than testable classes than manufactory pattern. Please come across the reply for a to a greater extent than detailed give-and-take on this topic.
113) Difference betwixt Adapter in addition to Decorator pattern? (answer)
Though the construction of Adapter in addition to Decorator pattern is similar, the departure comes on the intent of each pattern. The adapter pattern is used to span the gap betwixt 2 interfaces, but Decorator pattern is used to add together novel functionality into the degree without the modifying existing code.
114) Difference betwixt Adapter in addition to Proxy Pattern? (answer)
Similar to the previous question, the departure betwixt Adapter in addition to Proxy patterns is inward their intent. Since both Adapter in addition to Proxy pattern encapsulate the degree which genuinely does the job, thus number inward the same structure, but Adapter pattern is used for interface conversion piece the Proxy pattern is used to add together an extra flat of indirection to back upwardly distribute, controlled or intelligent access.
115) What is Template method pattern? (answer)
Template pattern provides an outline of an algorithm in addition to lets you lot configure or customize its steps. For examples, you lot tin terminate sentiment a sorting algorithm every bit a template to sort object. It defines steps for sorting but allow you lot configure how to compare them using Comparable or something similar inward some other language. The method which outlines the algorithms is also known every bit template method.
116) When do you lot utilisation Visitor pattern pattern? (answer)
The visitor pattern is a solution of occupation where you lot demand to add together functioning on a degree hierarchy but without touching them. This pattern uses double dispatch to add together some other flat of indirection.
117) When do you lot utilisation Composite pattern pattern? (answer)
Composite pattern pattern arranges objects into tree structures to stand upwardly for part-whole hierarchies. It allows clients care for private objects in addition to container of objects uniformly. Use Composite pattern when you lot desire to stand upwardly for part-whole hierarchies of objects.
118) The departure betwixt Inheritance in addition to Composition? (answer)
Though both allows code reuse, Composition is to a greater extent than flexible than Inheritance because it allows you lot to switch to some other implementation at run-time. Code written using Composition is also easier to examine than code involving inheritance hierarchies.
119) Describe overloading in addition to overriding inward Java? (answer)
Both overloading in addition to overriding allow you lot to write 2 methods of unlike functionality but with the same name, but overloading is compile fourth dimension activity piece overriding is run-time activity. Though you lot tin terminate overload a method inward the same class, but you lot tin terminate alone override a method inward tike classes. Inheritance is necessary for overriding.
120) The departure betwixt nested populace static degree in addition to a top flat degree inward Java? (answer)
You tin terminate receive got to a greater extent than than i nested populace static degree within i class, but you lot tin terminate alone receive got i top-level populace degree inward a Java origin file in addition to its call must last same every bit the call of Java origin file.
121) Difference betwixt Composition, Aggregation in addition to Association inward OOP? (answer)
If 2 objects are related to each other, they are said to last associated with each other. Composition in addition to Aggregation are 2 forms of association inward object-oriented programming. The composition is stronger association than Aggregation. In Composition, i object is OWNER of some other object piece inward Aggregation i object is only USER of some other object. If an object Influenza A virus subtype H5N1 is composed of object B in addition to so B doesn't be if Influenza A virus subtype H5N1 ceased to exists, but if object Influenza A virus subtype H5N1 is only an aggregation of object B in addition to so B tin terminate exists fifty-fifty if Influenza A virus subtype H5N1 ceased to exist.
122) Give me an representative of pattern pattern which is based upon opened upwardly closed principle? (answer)
This is i of the practical questions I enquire experienced Java programmer. I await them to know well-nigh OOP pattern principles every bit good every bit patterns. Open closed pattern regulation asserts that your code should last opened upwardly for extension but closed for modification. Which agency if you lot desire to add together novel functionality, you lot tin terminate add together it easily using the novel code but without touching already tried in addition to tested code. There are several pattern patterns which are based upon opened upwardly closed pattern regulation e.g. Strategy pattern if you lot demand a novel strategy, only implement the interface in addition to configure, no demand to modify center logic. One working representative is Collections.sort() method which is based on Strategy pattern in addition to follows the open-closed principle, you lot don't modify sort() method to sort a novel object, what you lot do is only implement Comparator inward your ain way.
123) Difference betwixt Abstract manufactory in addition to Prototype pattern pattern? (answer)
This is the exercise inquiry for you, If you lot are feeling bored only reading in addition to itching to write something, why non write the reply to this question. I would beloved to come across an representative the, which should reply where you lot should utilisation the Abstract manufactory pattern in addition to where is the Prototype pattern is to a greater extent than suitable.
124) When do you lot utilisation Flyweight pattern? (answer)
This is some other pop inquiry from the pattern pattern. Many Java developers with four to half dozen years of experience know the Definition but failed to give whatever concrete example. Since many of you lot powerfulness non receive got used this pattern, it's improve to appear examples from JDK. You are to a greater extent than probable receive got used them earlier in addition to they are piece of cake to call upwardly every bit well. Now let's come across the answer.
Flyweight pattern allows you lot to portion object to back upwardly large numbers without genuinely creating likewise many objects. In club to use Flyweight pattern, you lot demand to brand your object Immutable so that they tin terminate last safely shared. String puddle in addition to puddle of Integer in addition to Long object inward JDK are goodness examples of Flyweight pattern.
Miscellaneous Java Interview Questions
It contains XML Processing inward Java Interview question, JDBC Interview question, Regular expressions Interview questions, Java Error in addition to Exception Interview Questions, Serialization,125) The departure betwixt nested static degree in addition to top flat class? (answer)
One of the key questions from Java basics. I enquire this inquiry alone to junior Java developers of 1 to 2 years of experience every bit it's likewise piece of cake for an experience Java programmers. The reply is simple, a populace top flat degree must receive got the same call every bit the call of the origin file, at that spot is no such requirement for nested static class. Influenza A virus subtype H5N1 nested degree is e'er within a top flat degree in addition to you lot demand to utilisation the call of the top-level degree to refer nested static degree e.g. HashMap.Entry is a nested static class, where HashMap is a top flat degree in addition to Entry is nested static class.
126) Can you lot write a regular facial expression to depository fiscal establishment check if String is a number? (solution)
If you lot are taking Java interviews in addition to so you lot should enquire at to the lowest degree i inquiry on the regular expression. This clearly differentiates an average programmer with a goodness programmer. Since i of the traits of a goodness developer is to know tools, regex is the best tool for searching something inward the log file, preparing reports etc. Anyway, reply to this inquiry is, a numeric String tin terminate alone incorporate digits i.e. 0 to nine and + in addition to - sign that likewise at start of the String, past times using this information you lot tin terminate write next regular facial expression to depository fiscal establishment check if given String is number or not
127) The departure betwixt checked in addition to unchecked Exception inward Java? (answer)
checked exception is checked past times the compiler at compile time. It's mandatory for a method to either handgrip the checked exception or declare them inward their throws clause. These are the ones which are a sub degree of Exception but doesn't descend from RuntimeException. The unchecked exception is the descendant of RuntimeException in addition to non checked past times the compiler at compile time. This inquiry is at nowadays becoming less pop in addition to you lot would alone let on this with interviews with small-scale companies, both investment banks in addition to startups are moved on from this question.
128) The departure betwixt throw in addition to throws inward Java? (answer)
the throw is used to genuinely throw an instance of java.lang.Throwable class, which agency you lot tin terminate throw both Error in addition to Exception using throw keyword e.g.
throw new IllegalArgumentException("size must last multiple of 2")
On the other hand, throws is used every bit constituent of method annunciation in addition to signals which form of exceptions are thrown past times this method so that its caller tin terminate handgrip them. It's mandatory to declare whatever unhandled checked exception inward throws clause inward Java. Like the previous question, this is some other oft asked Java interview inquiry from errors in addition to exception theme but likewise piece of cake to answer.
129) The departure betwixt Serializable in addition to Externalizable inward Java? (answer)
This is i of the oft asked questions from Java Serialization. The interviewer has been asking this inquiry since the 24-hour interval Serialization was introduced inward Java, but yet alone a few goodness candidate tin terminate reply this inquiry with some confidence in addition to practical knowledge. Serializable interface is used to brand Java classes serializable so that they tin terminate last transferred over network or their Earth tin terminate last saved on disk, but it leverages default serialization built-in JVM, which is expensive, frail in addition to non secure. Externalizable allows you lot to fully command the Serialization process, specify a custom binary format in addition to add together to a greater extent than security measure.
130) The departure betwixt DOM in addition to SAX parser inward Java? (answer)
Another mutual Java inquiry but from XML parsing topic. It's rather unproblematic to reply in addition to that's why many interviewers prefers to enquire this inquiry on the telephonic round. DOM parser loads the whole XML into retention to create a tree based DOM model which helps it rapidly locate nodes in addition to brand a alter inward the construction of XML piece SAX parser is an trial based parser in addition to doesn't charge the whole XML into memory. Due to this argue DOM is faster than SAX but require to a greater extent than retention in addition to non suitable to parse large XML files.
131) Tell me 3 features introduced on JDK 1.7? (answer)
This is i of the goodness questions I enquire to depository fiscal establishment check whether the candidate is aware of recent evolution inward Java technology infinite or not. Even though JDK seven was non a large bang release similar JDK five or JDK 8, it even so has a lot of goodness characteristic to count on e.g. try-with-resource statements, which gratis you lot from closing streams in addition to resources when you lot are done with that, Java automatically closes that. Fork-Join puddle to implement something similar the Map-reduce pattern inward Java. Allowing String variable in addition to literal into switch statements. Diamond operator for improved type inference, no demand to declare generic type on the right-hand side of variable annunciation anymore, results inward to a greater extent than readable in addition to succinct code. Another worth noting characteristic introduced was improved exception treatment e.g. allowing you lot to take take hold of multiple exceptions inward the same take take hold of block.
132) Tell me five features introduced inward JDK 1.8? (answer)
This is the follow-up inquiry of the previous one. Java 8 is path breaking release inward Java's history, hither are the top five features from JDK 8 release
- Lambda expression, which allows you lot transcend an anonymous constituent every bit object.
- Stream API, take payoff of multiple cores of modern CPU in addition to allows you lot to write succinct code.
- Date in addition to Time API, finally you lot receive got a solid in addition to piece of cake to utilisation appointment in addition to fourth dimension library right into JDK
- Extension methods, at nowadays you lot tin terminate receive got static in addition to default method into your interface
- Repeated annotation, allows you lot apply the same musical note multiple times on a type
133) What is the departure betwixt Maven in addition to ANT inward Java? (answer)
Another keen inquiry to depository fiscal establishment check the all circular cognition of Java developers. It's piece of cake to reply questions from center Java but when you lot enquire well-nigh setting things up, edifice Java artifacts, many Java software engineer struggles. Coming dorsum to the reply of this question, Though both are build tool in addition to used to create Java application build, Maven is much to a greater extent than than that. It provides criterion construction for Java projection based upon "convention over configuration" concept in addition to automatically deal dependencies (JAR files on which your application is dependent) for Java application. Please come across the reply for to a greater extent than differences betwixt Maven in addition to ANT tool.
That's all guys, lots of Java Interview questions? isn't it? I am certainly if you lot tin terminate reply this listing of Java questions you lot tin terminate easily crevice whatever center Java or advanced Java interview. Though I receive got non included questions from Java EE or J2EE topics e.g. Servlet, JSP, JSF, JPA, JMS, EJB or whatever other Java EE technology or from major spider web frameworks similar Spring MVC, Struts 2.0, Hibernate or both SOAP in addition to RESTful spider web services, it's even so useful for Java developers preparing for Java spider web developer position, because every Java interview starts with questions from fundamentals in addition to JDK API. If you lot think, I receive got missed whatever pop Java inquiry hither in addition to you lot think it should last inward this listing in addition to so experience gratis to propose me. My destination is to create the best listing of Java Interview Questions with latest in addition to greatest inquiry from recent interviews.
Further Learning
Complete Java Masterclass
see here)
Recommended Books for Java Programmers
If you lot are looking for some goods to ready for your Java Interviews, You tin terminate take a appear at next books to comprehend both theory in addition to coding questions:
- nine Must Read Books for Java Developers (see book)
- five Java Performance Tuning Books for Experienced Programmers (see book)
- five Good Books for Java JEE Interviews (see book)
- five Books to larn Data Structure in addition to Algorithms (see book)
- five Hibernate Books for Java Developers (see book)
- five Spring Books for Java Developers (see book)
References
- The Java Virtual Machine Spefication (see here)
- The Java linguistic communication Specification (Java SE 8) (see here)
- Java SE 8 API Specification (see here)
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