How HashMap works in Java

HashMap  works on principle of hashing, we have put() and get() method for storing and retrieving object form HashMap .When we pass an both key and value to put() method to store on HashMap , it uses key object hashcode() method to calculate hashcode and they by applying hashing on that hashcode it identifies bucket location for storing value object. While retrieving it uses key object equals method to find out correct key value pair and return value object associated with that key. HashMap  uses linked list in case of collision and object will be stored in next node of linked list.

Also HashMap  stores both key and value tuple in every node of linked list

What will happen if two different HashMap  key objects have same hashcode?

They will be stored in same bucket but no next node of linked list. And keys equals () method will be used to identify correct key value pair in HashMap .

Have you used HashMap before" or  "What is HashMap? Why do we use it 

Almost everybody answers this with yes and then interviewee keep talking about common facts about HashMap like HashMap accept null while Hashtable doesn't, HashMap is not synchronized, HashMap is fast and so on along with basics like its stores key and value pairs etc. This shows that person has used HashMap  and quite familiar with the functionality HashMap offers but interview takes a sharp turn from here and next set of follow-up questions gets more detailed about fundamentals involved with HashMap in Java . Interviewer struck back with questions like

"Do you Know how HashMap works in Java" or "How does get () method of HashMap works in Java"

And then you get answers like I don't bother its standard Java API, you better look code on Java source or Open JDK; I can find it out in Google at any time etc. But some interviewee definitely answer this and will say "HashMap works on principle of hashing, we have put(key, value) and get(key) method for storing and retrieving Objects from HashMap. When we pass Key and Value object  toput() method on Java HashMap, HashMap implementation calls hashCode method on Key object and applies returned hashcode into its own hashing function to find a bucket location for storing Entry object, important point to mention is that HashMap in Java stores both key and value object as Map.Entry in bucket which is essential to understand the retrieving logic. If people fails to recognize this and say it only stores Value in the bucket they will fail to explain the retrieving logic of any object stored in Java HashMap . This answer is very much acceptable and does make sense that interviewee has fair bit of knowledge on how hashing works and how HashMap  works in Java. But this is just start of story and confusion increases when you put interviewee on scenarios faced by Java developers on day by day basis. Next question could be about collision detection and collision resolution in Java HashMap  e.g. 

"What will happen if two different objects have same hashcode?"

Now from here onwards real confusion starts, Some time candidate will say that since hashcode is equal, both objects are equal and HashMap  will throw exception or not store them again etc, Then you might want to remind them about equals() and hashCode() contract  that two unequal object in Java can have same hashcode. Some will give up at this point and few will move ahead and say "Since hashcode is same, bucket location would be same and collision will occur in HashMap, Since HashMap use LinkedList to store object, this entry (object of Map.Entry comprise key and value )  will be stored in LinkedList. Great this answer make sense though there are many collision resolution methods available this is simplest and HashMap in Java does follow this. But story does not end here and interviewer asks

"How will you retrieve Value object  if two Keys will have same hashcode?"

Interviewee will say we will call get() method and then HashMap uses Key Object's hashcode to find out bucket location and retrieves Value object but then you need to remind him that there are two Value objects are stored in same bucket , so they will say about traversal in LinkedList until we find the value object , then you ask how do you identify value object because you don't  have value object to compare ,Until they know that HashMap  stores both Key and Value in LinkedList node or as Map.Entry they won't be able to resolve this issue and will try and fail.

But those bunch of people who remember this key information will say that after finding bucket location , we will call keys.equals() method to identify correct node in LinkedList and return associated value object for that key in Java HashMap . Perfect this is the correct answer.

In many cases interviewee fails at this stage because they get confused between hashCode() and equals() or keys and values object in Java HashMap  which is pretty obvious because they are dealing with the hashcode() in all previous questions and equals() come in picture only in case of retrieving value object from HashMap in Java. Some good developer point out here that using immutable,final object with proper equals() and hashcode() implementation would act as perfect Java HashMap  keys and improve performance of Java HashMap  by reducing collision. Immutability also allows caching there hashcode of different keys which makes overall retrieval process very fast and suggest that String and various wrapper classes e.g. Integer very good keys in Java HashMap.

Now if you clear this entire Java HashMap interview,  You will be surprised by this very interesting question "What happens On HashMap in Java if the size of the HashMap  exceeds a given threshold defined by load factor ?". Until you know how HashMap  works exactly you won't be able to answer this question. If the size of the Map exceeds a given threshold defined by load-factor e.g. if load factor is .75 it will act to re-size the map once it filled 75%. Similar to other collection classes like ArrayList,  Java HashMap re-size itself by creating a new bucket array of size twice of previous size of HashMap , and then start putting every old element into that new bucket array. This process is called rehashing because it also applies hash function to find new bucket location. 

If you manage to answer this question on HashMap in Java you will be greeted by "do you see any problem with resizing of HashMap  in Java" , you might not be able to pick the context and then he will try to give you hint about multiple thread accessing the Java HashMap and potentially looking for race condition on HashMap  in Java. 

So the answer is Yes there is potential race condition exists while resizing HashMap in Java, if two thread at the same time found that now HashMap needs resizing and they both try to resizing. on the process of resizing of HashMap in Java , the element in bucket which is stored in linked list get reversed in order during there migration to new bucket because java HashMap  doesn't append the new element at tail instead it append new element at head to avoid tail traversing. If race condition happens then you will end up with an infinite loop. Though this point you can potentially argue that what the hell makes you think to use HashMap  in multi-threaded environment to interviewer

Few more question on HashMap

1) Why String, Integer and other wrapper classes are considered good keys ?

String, Integer and other wrapper classes are natural candidates of HashMap key, and String is most frequently used key as well because String is immutable and final,and overrides equals and hashcode() method. Other wrapper class also shares similar property. Immutabiility is required, in order to prevent changes on fields used to calculate hashCode() because if key object return different hashCode during insertion and retrieval than it won't be possible to get object from HashMap. Immutability is best as it offers other advantages as well like thread-safety, If you can  keep your hashCode same by only making certain fields final, then you go for that as well. Since equals() and hashCode() method is used during reterival of value object from HashMap, its important that key object correctly override these methods and follow contact. If unequal object return different hashcode than chances of collision will be less which subsequently improve performance of HashMap.

2) Can we use any custom object as key in HashMap ?

This is an extension of previous questions. Ofcourse you can use any Object as key in Java HashMap provided it follows equals and hashCode contract and its hashCode should not vary once the object is inserted into Map. If custom object is Immutable than this will be already taken care because you can not change it once created.

3) Can we use ConcurrentHashMap in place of Hashtable ?

This is another question which getting popular due to increasing popularity of ConcurrentHashMap. Since we know Hashtable is synchronized but ConcurrentHashMap provides better concurrency by only locking portion of map determined by concurrency level. ConcurrentHashMap is certainly introduced as Hashtable and can be used in place of it but Hashtable provide stronger thread-safety than ConcurrentHashMap.