Top 21 Coffee Hashmap Interview Questions As Well As Answers

The java.util.HashMap is ane of the workhorses of JDK. Along alongside ArrayList, it is ane of the most used classes from Java's collection framework. There is hardly a real-world Java project, where I haven't seen the occupation of HashMap. It is an implementation of hash tabular array information construction as well as it's non a surprise that HashMap is so useful, as somebody has rightly said, "if you lot could receive got simply ane information structure, arrive a hash table". The hash tabular array information construction allows you lot to search for a value inwards O(1) fourth dimension if you lot receive got key. In Java, in that location are several implementations of hash tabular array information construction exists similar Hashtable, ConcurrentHashMap, LinkedHashMap, etc but HashMap is your full general role map.

Though, if you lot receive got a special demand you lot tin terminate occupation other hash tabular array implementations available inwards JDK. For example, if you lot desire to save the companionship of mapping as well as so you lot tin terminate consider using LinkedHashMap. If you lot desire to proceed mappings sorted as well as so you lot tin terminate occupation TreeMap, which is a sorted map implementation.

Similarly, if you lot demand a hash tabular array implementation which is thread-safe as well as tin terminate last used inwards a concurrent application without compromising the Scalability as well as so consider using a ConcurrentHashMap from JDK 5.

Btw, if you lot are novel to Java globe as well as JDK API inwards particular, I advise you lot to start snuff it through a comprehensive Java course of didactics similar The Complete Java Masterclass on Udemy. That volition non exclusively assist you lot to produce good on interviews but likewise assist you lot to sympathize the fundamentals better.




Java HashMap Interview Questions

Here is my listing of HashMap questions from Java Interviews. This listing includes questions based on the internal implementation of HashMap, the Map API, how you lot occupation HashMap as well as mutual best practices spell using HashMap inwards a Java application.


1. How does put() method of HashMap industrial plant inwards Java? (answer)
The put() method of HashMap industrial plant inwards the regulation of hashing. It is responsible for storing an object into the backend array. The hashcode() method is used inwards conjunction alongside a hash component to detect the right location for the object into the bucket. If a collision occurs as well as so the entry object which contains both key as well as value is added to a linked listing and that linked listing is stored into the bucket location.


2. What is the requirement for an object to last used as key or value inwards HashMap? (answer)
The key or value object must implement equals() as well as hashcode() method. The hash code is used when you lot insert the key object into the map spell equals are used when you lot effort to recollect a value from the map.


3. What volition hand off if you lot effort to shop a key which is already nowadays inwards HashMap? (answer)
If you lot shop an existing key inwards the HashMap as well as so it volition override the one-time value alongside the novel value as well as put() volition render the one-time value. There volition non last whatsoever exception or error.


4. Can you lot shop a zip key inwards Java HashMap? (answer)
Yes, HashMap allows ane zip key which is stored at the start location of bucket array e.g. bucket[0] = value. The HashMap doesn't telephone telephone hashCode() on zip key because it volition throw NullPointerException, therefore when a user telephone telephone get() method alongside zip as well as so the value of the start index is returned.

5. Can you lot shop a zip value within HashMap inwards Java? (answer)
Yes, HashMap likewise allows zip value, you lot tin terminate shop as many zip values as you lot desire as shown inwards the hashmap representative postal service inwards this blog.


6. How does HashMap grip collisions inwards Java? (answer)
The java.util.HashMap uses chaining to grip collisions, which agency novel entries, an object which contains both key as well as values, are stored inwards a linked listing along alongside the existing value as well as and so that linked listing is stored inwards the bucket location.

In the worst case, where all key has the same hashcode, your hash tabular array volition last turned into a linked listing as well as searching a value volition receive got O(n) fourth dimension as opposed to O(1) time.

If you lot desire to larn to a greater extent than nearly hash tabular array information structure, I advise you lot consult a adept information construction as well as algorithm course of didactics like Data Structures as well as Algorithms: Deep Dive Using Java on Udemy, which non exclusively encompass basic information construction similar array, linked list, binary tree, as well as hash tabular array but likewise advanced concepts similar O(n) sorting algorithms, Radix sort, Counting sort, etc. 

 it is ane of the most used classes from Java Top 21 Java HashMap Interview Questions as well as Answers



7. Which information construction HashMap represents? (answer)
The HashMap is an implementation of hash tabular array information construction which is idle for mapping ane value to other similar id to refer as you lot tin terminate search for value inwards O(1) fourth dimension if you lot receive got the key.


8. Which information construction is used to implement HashMap inwards Java? (answer)
Even though HashMap represents a hash table, it is internally implemented yesteryear using an array as well as linked listing information construction inwards JDK.  The array information construction is used as a bucket spell a linked list is used to shop all mappings which state inwards the same bucket. From Java 8 onwards, the linked listing is dynamically replaced yesteryear binary search tree, ane time a number of elements inwards the linked listing cross a for certain threshold to improve performance.


9. Can you lot shop a duplicate key inwards HashMap? (answer)
No, you lot cannot insert duplicate keys inwards HashMap, it doesn't allow duplicate keys. If you lot effort to insert an existing key alongside novel or same value as well as so it volition override the one-time value but size of HashMap volition non modify i.e. it volition stay the same. This is ane of the argue when you lot acquire all keys from the HashMap yesteryear calling keySet() it returns a Set, non a Collection because Set doesn't allow duplicates.


10. Can you lot shop the duplicate value inwards Java HashMap? (answer)
Yes, you lot tin terminate pose duplicate values inwards HashMap of Java. It allows duplicate values, that's why when you lot recollect all values from the Hashmap yesteryear calling values() method it returns a Collection as well as non Set. Worth noting is that it doesn't render List because HashMap doesn't render whatsoever ordering guarantee for key or value.

If you lot desire to explore, you lot tin terminate likewise see answer)
No, HashMap is non thread-safe inwards Java. You should non percentage a HashMap alongside multiple threads if ane or to a greater extent than thread is modifying the HashMap e.g. inserting or removing a map. Though, you lot tin terminate easily percentage a read-only HashMap.


12. What volition hand off if you lot occupation HashMap inwards a multithreaded Java application? (answer)
If you lot occupation HashMap inwards a multithreaded surround inwards such a way that multiple threads structurally modify the map e.g. add, take away or modify mapping as well as so the internal information construction of HashMap may acquire corrupt similar to a greater extent than or less links may snuff it missing, to a greater extent than or less may signal to wrong entries as well as the map itself may snuff it completely useless. Hence, it is advised non to occupation HashMap inwards the concurrent application, instead, you lot should occupation a thread-safe map e.g. ConcurrentHashMap or Hashtable.


13. What are different ways to iterate over HashMap inwards Java? (answer)
Here are to a greater extent than or less of the ways to iterate over HashMap inwards Java:
yesteryear using keySet as well as iterator
yesteryear using entrySet as well as iterator
yesteryear using entrySet as well as enhanced for loop
yesteryear using keySet as well as get() method

You tin terminate run into this article for an representative of each of these ways to traverse a HashMap inwards Java.


14. How produce you lot take away a mapping spell iterating over HashMap inwards Java? (answer)
Even though HashMap provides remove() method to take away a key as well as a key/value pair, you lot cannot occupation them to take away a mapping spell traversing a HashMap, instead, you lot demand to occupation the Iterator's take away method to take away a mapping as shown inwards the next example:

Iterator itr = map.entrySet().iterator();  while(itr.hasNext()){   Map.Entry electrical flow = itr.next();    if(current.getKey().equals("matching"){      itr.remove(); // this volition take away the electrical flow entry.   } }

You tin terminate run into that nosotros receive got used Iterator.remove() method to take away the electrical flow entry spell traversing the map. See this article to larn to a greater extent than nearly it.


15. In which companionship mappings are stored inwards HashMap? (answer)
Random companionship because HashMap doesn't render whatsoever ordering guarantee for keys, values, or entries. When you lot iterate over a HashMap, you lot may acquire the different companionship every fourth dimension you lot iterate over it.




16. Can you lot variety HashMap inwards Java? (answer)
No, you lot cannot variety a HashMap because dissimilar List it is non an ordered collection. Albeit, you lot tin terminate variety contents of HashMap yesteryear keys, values or yesteryear entries yesteryear sorting as well as and so storing the trial into an ordered map similar LinkedHashMap or a sorted map e.g. TreeMap.


17. What is the charge factor inwards HashMap? (answer)
Influenza A virus subtype H5N1 charge factor is a number which controls the resizing of HashMap when a number of elements inwards the HashMap cross the charge factor e.g. if the charge factor is 0.75 as well as when becoming to a greater extent than than 75% amount as well as so resizing trigger which involves array copy.


18. How does resizing happens inwards HashMap? (answer)
The resizing happens when the map becomes amount or when the size of the map crosses the charge factor. For example, if the charge factor is 0.75 as well as and so snuff it to a greater extent than than 75% amount as well as so resizing trigger which involves array copy. First, the size of the bucket is doubled as well as and so one-time entries are copied into a novel bucket.


19. How many entries you lot tin terminate shop inwards HashMap? What is the maximum limit? (answer)
There is no maximum boundary for HashMap, you lot tin terminate shop as many entries as you lot desire because when you lot run out of the bucket, entries volition last added to a linked listing which tin terminate back upwards an infinite number of entries, of course of didactics until you lot exhaust all the retention you lot have.

Btw, the size() method of HashMap render an int, which has a limit, ane time a number of entries cross the limit, size() volition overflow as well as if your programme relies on that as well as so it volition break.

This number has been addressed inwards JDK 8 yesteryear introducing a novel method called mappingCount() which returns a long value. So, you lot should occupation mappingCount() for large maps. See Java SE 8 for Really Impatient to larn to a greater extent than nearly novel methods introduced inwards existing interfaces inwards JDK 8.

 it is ane of the most used classes from Java Top 21 Java HashMap Interview Questions as well as Answers


21. What is the departure betwixt capacity as well as size of HashMap inwards Java? (answer)
The capacity denotes how many entries HashMap tin terminate shop as well as size denotes how many mappings or key/value brace is currently present.


21. What volition hand off if 2 different keys of HashMap render same hashcode()? (answer)
If 2 keys of HashMap render same hash code as well as so they volition destination upwards inwards the same bucket, therefore collision volition occur. They volition last stored inwards a linked listing together.


That's all nearly to a greater extent than or less of the of import Java HashMap interview questions. I receive got tried to reply them as well, but if you lot disagree alongside an reply as well as so experience gratis to comment. Since HashMap is a rattling of import course of didactics inwards Java as well as as of import from Java interview signal of view, it pays to sympathize this course of didactics as well as its implementation inwards deep.

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  • These questions volition non exclusively assist you lot to sympathize HashMap ameliorate but likewise encourage you lot to detect out to a greater extent than nearly HashMap, it's Java implementation as well as hash tabular array information construction inwards general. If you lot receive got whatsoever other HashMap based Java questions, which was asked to you lot inwards an interview, experience gratis to percentage alongside us.

    P. S. - If you lot are preparing for Java Interviews as well as looking for to a greater extent than or less interesting questions for practise as well as so you lot tin terminate likewise cheque out this Java Interview Guide: 200+ Interview Questions as well as Answers course, which contains to a greater extent than than 200+ real-world questions from Java interviews as well as their explanation.

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