Difference between HashMap vs IdentityHashMap in Java?

The IdentityHashMap is one of the lesser known Map implementation from JDK. Unlike general purposes Map implementations like HashMap and LinkedHashMap, it is very special and it's internal working is quite different than HashMap. The main difference between IdentityHashMap and HashMap in Java is that former uses equality operator (==) instead of equals() method to compare keys. Which means you need the same key object to retrieve the value from IdentityHashMap, you cannot retrieve values by using another key which is logically equal to previous key. Another important difference between HashMap and IdentityHashMap is that IdentityHashMap doesn't use hashCode() method instead it uses System.identityHashCode() method. This is a significant difference because now you can use mutable objects as key in Map whose hash code are likely to change when the mapping is stored inside IdentityHashMap.

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