Top Ten Concurrenthashmap Questions From Coffee Interviews

The ConcurrentHashMap course of pedagogy component of concurrent collections bundle added on JDK 1.5 which contains utility classes like BlockingQueue, CopyOnWriteArrayList, CopyOnWriteArraySet etc. It is a replacement of synchronized hash-based map implementations e.g. Hashtable in addition to synchronized HashMap. It implements Map in addition to ConcurrentMap (a sub-interface of Map) interface which allows y'all to shop key-value pairs. The course of pedagogy is similar to HashMap or Hashtable merely it's to a greater extent than scalable and the correct fit for concurrent Java application. Unlike Hashtable which achieves its thread-safety past times compromising the scalability, ConcurrentHashMap uses advanced techniques e.g. dividing the map into segments to rest thread-safe in addition to scalable at the same time.

Because of its performance in addition to scalability equally good equally thread-safety, it is the most pop selection of Map inwards concurrent Java applications.

In general, replacing synchronized collections alongside a concurrent collection tin forcefulness out dramatically improve the scalability of your Java application alongside a trivial risk, equally advised inwards of the classic Java mass of all time, the Java Concurrency inwards Practice book past times Brian Goetz.

If y'all receive got non read this mass so I highly recommend it to every unmarried Java programmer. Many of y'all may abide by it hard when y'all read it the commencement fourth dimension merely persist alongside it in addition to y'all volition reap the dividend inwards your career equally Java programmer.

If y'all withal abide by concepts hard to empathize y'all tin forcefulness out besides bring help from Heinz Kabutz's awesome course Java Concurrency inwards Practice Bundle, which is based on this book. Heinz is i of the Java champion in addition to fantabulous teacher who has gifted teaching skills. He explains hard concepts inwards such an slow linguistic communication in addition to examples which y'all tin forcefulness out correlate.

Anyway, let's start alongside this listing of Java interview questions based upon ConcurrentHashMap course of pedagogy in addition to concepts roughly it.

Java ConcurrentHashMap Interview Questions alongside Answers

Here are some of the best in addition to oft asked Java ConcurrentHashMap interview questions. These questions are collected from the existent interview, hence, don't hold out surprised if y'all receive got already seen them during interviews.

These questions volition non solely help y'all to hit good on interviews merely besides encourage y'all to larn to a greater extent than close the concurrent hash map, which volition eventually help y'all inwards your solar daytime to solar daytime programming job.


1. What is ConcurrentHashMap inwards Java? (answer)
The java.util.concurrent.ConcurrentHashMap is a concurrent collection course of pedagogy added on JDK 1.5 equally a replacement of synchronized hash-based map implementations e.g. Hashtable in addition to synchronized HashMap. They offering amend performance in addition to scalability over their synchronized counterpart alongside trivial risk.


2. Does ConcurrentHashMap thread-safe inwards Java? (answer)
Yes, ConcurrentHashMap is thread-safe inwards Java, which agency 2 thread tin forcefulness out modify the map without damaging its internal information construction e.g. array in addition to linked list. If y'all compare this to HashMap, which is non thread-safe, exposing HashMap to multiple threads may harm internal information structure in addition to may homecoming the map completely useless, where many links may become missing or pointing to incorrect elements.


3. How does ConcurrentHashMap attain thread-safety? (answer)
The java.util.ConcurrentHashMap achieves thread-safety past times dividing the map into segments in addition to locking solely the segment which requires instead of locking the whole map. So, yes, it achieves thread-safety using locking merely it performs amend because dissimilar HashMap, it never locks the whole map. This technique is besides known equally lock stripping.

If y'all desire to larn to a greater extent than close it, y'all tin forcefulness out besides bring a facial expression at answer)
Yes, ConcurrentHashMap allows concurrent read without locking equally reading performance doesn't require locking or thread-safety.


5. Can i thread read in addition to other writes on ConcurrentHashMap at the same time? (answer)
Yes, it's possible for a small-scale reveal of the writer. For example, if a write performance is modifying i segment of ConcurrentHashmap in addition to read performance is happening on other segments so a reader volition non block, merely if reader thread is besides trying to read from the same segment than it volition block until the author is done.


6. How does ConcurrentHashMap piece of work internally? (answer)
The java.util.ConcurrentHashMap industrial plant similar to HashMap when it comes to storing key/value pairs in addition to retrieving values. The solely deviation inwards its implementation comes from concurrency perspective in addition to how it achievers thread-safety. It divides the map into several segments, past times default 16, besides known equally synchronization level.

Because of this, concurrent get(), put(), contains() performance is possible because it never locks the whole map merely solely the relevant segment is locked. Which agency readers tin forcefulness out access the map concurrency alongside writers in addition to a express reveal of writers tin forcefulness out modify the map concurrently. The upshot is amend throughput in addition to Scalability.

You tin forcefulness out farther run across The Complete Java MaseterClasss course of pedagogy on Udemy for details on the implementation of ConcurrentHashMap class. This course of pedagogy is lately updated for Java 11, the latest Java version equally well.

Here is a diagram which explains how a segment looks similar inwards a ConcurrentHashMap of Java, basically it's nil merely a mini hash tabular array alongside a bucket in addition to a linked listing of hash entries inwards instance of collision:

Since the Iterator returned past times ConcurrentHashMap is weakly consistent, the recent concurrency modification may or may non hold out visible to it. There is no guarantee offered on such operation.

I  besides propose y'all joining course Java Concurrency inwards Practice Bundle, to larn to a greater extent than close how concurrency is handled past times ConcurrentHashMap inwards Java.



7. How hit y'all atomically update a value inwards ConcurrentHashMap? (answer)
If y'all desire to atomically update an existing value inwards ConcurrentHashMap, y'all tin forcefulness out run the replace() component of concurrentHashMap.

It accepts both one-time value in addition to novel value in addition to solely updates the map if the existing value inwards the map matches alongside the one-time value provided, this agency the map is non concurrently modified during its call.

If the existing value is changed in addition to non matches alongside the one-time value so supplant neglect alongside returning false. You tin forcefulness out run telephone outcry upward the replace() method inwards spell loop until y'all succeed, equally shown below:

ConcurrentMap<String, Long> populationByCities = new ConcurrentHashMap<>(); do{   Long currentValue = populationByCities.get("New York");   Long newValue = currentValue == null ? 1 : currentValue + 1; }while(!populationByCities.replace("New York", currentValue, newValue));

You tin forcefulness out besides see Java SE 8 for Really Impatient for some code examples of atomically updating an existing value inwards ConcurerntHashMap.

8. How hit y'all take a mapping spell iterating over ConcurrentHashMap? (answer)
You tin forcefulness out run an Iterator to take a mapping from ConcurrentHashMap inwards Java equally shown below:

Map<String, Integer> bookAndPrice = novel ConcurrentHashMap<>(); bookAndPrice.put("Effective Java", 42); bookAndPrice.put("Head First Java", 29); bookAndPrice.put("Java Concurrency inwards Practice", 33); bookAndPrice.put("Head First Design Patterns", 41);  System.out.println("before removing : " + bookAndPrice); Iterator<String> iterator = bookAndPrice.keySet().iterator();  while(iterator.hasNext()){   if(iterator.next().contains("Java")){   iterator.remove(); } }   System.out.println("after removing : " + bookAndPrice);  Output earlier removing : {Java Concurrency inwards Practice=33,  Head First Design Patterns=41, Effective Java=42, Head First Java=29} after removing : {Head First Design Patterns=41}

9. Does Iterator of ConcurrentHashMap is fail-safe or fail-fast? (answer)
Iterator of ConcurrentHashMap is a fail-safe iterator which agency it volition non throw a ConcurrentModificationException, thus, eliminating the demand to lock the map during iteration.

The Iterator returned past times ConcurrentHashMap are besides weakly consistent which agency if the Map is modified during iteration, it may or may non reverberate the recent modification. Generally, it creates a re-create of collection earlier iterating.



10. What volition spill out if y'all add together a novel mapping inwards ConcurrentHashMap spell i thread is iterating over it? (answer)
This is i of the tricky questions related to ConcurrentHashMap. Since iterator's of ConcurrentHashMap are weekly consistent in addition to fail-safe they volition non neglect alongside ConcurrentModificationException merely it's besides possible that they won't run across whatever modification i time iteration started. Even though it's implementation dependent, JDK by in addition to large creates a separate re-create of ConcurrentHashMap for iteration, instead of iterating over master copy copy.


11. Can y'all travel past times an object of ConcurrentHahsMap when a Map is expected? (answer)
Yes because ConcurrentHashMap implements java.util.concurrent.ConcurrentMap interface, which extends java.util.Map interface, thence ConcurrentHashMap IS-A Map. Also, y'all tin forcefulness out shop an object of ConcurrentHashMap into a Map variable equally shown below:

Map<String, Integer> bookAndPrice = new ConcurrentHashMap<>();

Though, this means, y'all may non receive got access to methods declared inwards the java.util.concurrent.ConcurrentHashMap course of pedagogy e.g. forEachKey() or forEachValue() method added inwards Java 8.


That's all close some of the frequently asked questions close ConcurrentHashMap on Java interviews. These questions volition non solely help y'all to hit good on your project interview merely besides encourage y'all to larn to a greater extent than close ConcurrentHashMap.

H5N1 adept in addition to enterprise noesis of ConcurrentHashMap is expected from both junior in addition to senior Java developer given its importance in addition to usability inwards every Java application.

At to the lowest degree y'all should hold out comfortable alongside day-to-day operations alongside ConcurrentHashMap in addition to empathize how the internal implementation works, particularly when compared to other thread-safe map implementations e.g. Hashtable in addition to Synchronized HashMap.


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Thanks for reading this article so far. If y'all similar these interview questions in addition to abide by my explanations useful so delight part alongside your friends in addition to colleagues. If y'all receive got whatever inquiry or doubtfulness so delight drib a comment.

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