Difference between IdentityHashMap, WeakHashMap and EnumMap in Java

Apart from popular implementation like HashMap and LinkedHashMap, java.util.Map also has some specialized implementation classes e.g. IdentifyHashMap which uses == instead of equals() method for comparing keys, WeakHashMap which uses WeakReference to wrap the key object and a key/value mapping is removed when the key is no longer referenced from elsewhere, and EnumMap where keys are Enum constants. Many Java developer doesn't know about these special Map implementation classes and failed to take advantage of improved performance and feature offered by them. Their knowledge is also important from core Java interview perspective because Java interviewer put a huge focus on JDK API, particularly Java Collection framework and Java Concurrency framework which includes concurrent collections classes.

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