What is ConcurrentHashMap-identityHashMap-WeakHashMap ?

What is ConcurrentHashMap-identityHashMap-WeakHashMap ?

Q) What is ConcurrentHashMap? 

Ans) A concurrentHashMap is thread-safe implementation of Map interface. In this class put and remove method are synchronized but not get method. This class is different from Hashtable in terms of locking; it means that hashtable use object level lock but this class uses bucket level lock thus having better performance. 

Q) What is identityHashMap?

Ans) The IdentityHashMap uses == for equality checking instead of equals(). This can be used for both performance reasons, if you know that two different elements will never be equals and for preventing spoofing, where an object tries to imitate another.

Q) What is WeakHashMap?

Ans) A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently than other Map implementations.

Soft references act like a data cache. When system memory is low, the garbage collector can arbitrarily free an object whose only reference is a soft reference. In other words, if there are no strong references to an object, that object is a candidate for release. The garbage collector is required to release any soft references before throwing an OutOfMemoryException.

Weak references are weaker than soft references. If the only references to an object are weak references, the garbage collector can reclaim the memory used by an object at any time. There is no requirement for a low memory situation. Typically, memory used by the object is reclaimed in the next pass of the garbage collector