#Map_interface

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TreeMap	It contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class. It contains only unique elements. It cannot have a null key but can have multiple null values. It is non synchronized. It maintains ascending order.
HashMap	It contains values based on the key. It contains only unique keys. It may have one null key and multiple null values. It is non synchronized. It maintains no order. The initial default capacity of Java HashMap class is 16 with a load factor of 0.75.
Hashtable	A Hashtable is an array of a list. Each list is known as a bucket. The position of the bucket is identified by calling the hashcode() method. A Hashtable contains values based on the key. Java Hashtable class contains unique elements. Java Hashtable class doesn't allow null key or value. Java Hashtable class is synchronized. The initial default capacity of Hashtable class is 11 whereas loadFactor is 0.75.
ConCurrentHashmap

TreeMap         ConcurrentMap         UnmodifiableMap         Hashtable         ConcurrentHashMap         ConcurrentNavigableMap         ConcurrentSkipListMap         SortedMap         NavigableMap         HashMap         IdentityHashMap         WeakTreeMap