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HashMap VS Hash Table Linked Hash Map vs Identity Map

What is difference between HashMap and HashTable?
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HashMap Hashtable
========================================================================================================
-> No method present inside HashMap is synchronized. -> Every methods inside Hashtable is synchronized.
-> At a time multiple threads are allowed to operate on -> At a time only one thread is allowed to operate on
     HashMap object simultaneously and hence it is not a      Hashtable object and hence it is Threadsafe.
     Threadsafe.
-> Relatively Performance is high. -> Relatively Performance is low.
-> Null is allowed for both keys and values. -> Null is not allowed for both keys and values. Otherwise
     we will get NullPointerException.

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HashMap LinkedHashMap
========================================================================================================
-> Underlying data structure is Hashtable. -> Underlying data structure is combination of Hashtable
     and LinkedList.
-> Insertion order is not preserved. -> Insertion order is preserved.
-> Introduced in 1.2 version. -> Introduced in 1.4 version.

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IdentityHashMap
==================
-> It is exactly same as HashMap except the following difference.
-> In case of HashMap, JVM will use .equals(-) method to identify duplicate keys, which is meant for content comparison.
-> But in case of IdentityHashMap, JVM will use == operator to identify duplicate keys which is meant for reference comparison.


WeakHashMap
==================
-> It is exactly same as HashMap except the following difference.
-> In case of HashMap, if an object associated with HashMap then it is not eligible for garbage collection,
     eventhough it doesn't contain any external references. i.e., HashMap dominates Garbage Collector.
-> But in case of WeakHashMap, if an object doesn't contain any references then it is always eligible for
     Garbage Collector eventhough it is associated with WeakHashMap. ie., Garbage Collector dominates WeakHashMap.

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