Difference Between IdentityHashMap, WeakHashMap, and EnumMap in Java

The IdentityHashMap, WeakHashMap, and EnumMap all are the classes in java collection that implements the Map interface. But there are few differences exists between them.

1. IdentityHashMap: The IdentityHashMap implements the Map interface. It follows reference-equality in place of object-equality when comparing keys (and values). This class is used when the user requires the objects to be compared via reference. It is not synchronized and must be synchronized externally. The iterators in this class are fail-fast, throw ConcurrentModificationException in an attempt to modify while iterating.

Working of IdentityHashMap:

IdentityHashMap size : 5
Initial identity hash map: {10=Geeks, 40=Welcomes, 50=You, 30=Geeks, 20=4}
Key = 10, Value = Geeks
Key = 40, Value = Welcomes
Key = 50, Value = You
Key = 30, Value = Geeks
Key = 20, Value = 4

2. WeakHashMap: WeakHashMap is the implementation of the Map interface that stores only weak keys. In WeakHashMap, we can store only weak references of its key that allows a key-value pairs to be garbage collected when its key is no longer in ordinary use. WeakHashMap is the HashTable based implementation, but it is not synchronized. It allows you to store both null key and null values.

Working of WeakHashMap: 

WeakHashMap is : {5=you, 4=welcomes, 3=geeks, 2=4, 1=geeks}
Yes welcomes exist
Yes 3 exist
key Set : [5, 4, 3, 2, 1]
Values : [you, welcomes, geeks, 4, geeks]
Empty WeakHashMap: {}

3. EnumMap: EnumMap is a specialized implementation of the Map interface for enumeration types. It extends AbstractMap and implements the Map interface in Java. Few important features of EnumMap are as follows: 

• EnumMap class is a member of the Java Collections Framework and it is not synchronized.
• EnumMap is an ordered collection, and they are maintained in the natural order of their keys(the natural order of keys means the order on which enum constant are declared inside enum type)
• EnumMap is much faster than HashMap.
• All keys of each EnumMap instance must be keys of a same enum type.
• EnumMap doesn’t allow to insert null key if we try to insert the null key, it will throw NullPointerException.
• EnumMap is internally represented as arrays therefore it gives the better performance.

Working of EnumMap:

Size of EnumMap: 4
January 31
February 28
March 31
April 30

Difference between IdentityHashMap, WeakHashMap, and EnumMap: