» Have you used HashMap before in Java or What is HashMap? Why do we use it

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

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  to put() 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?”
how HashMap works internally in JavaInterviewee 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.


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.