Java collections comparison

	HashMap	TreeMap
Ordering	It doesn’t maintain ordering	TreeMap  elements are sorted according to the natural ordering of its elements . If TreeMap objects can not be sorted in natural order than use compareTo() method to sort the elements of TreeMap object.
Implementation	Hashing	Red black tree
Null key	Can store null key	No null key
Null values	Many null values	Many null values
Performance	Constant time O(1) get and put, fast in performance.	TreeMap provides guaranteed log(n) time cost for the get and put method. Slow in comparison
Comparison	Uses equals method	Uses compareTo method
Interface	Map	Navigational Map
Similarities	Not thread safe, shallow copy, fail fast iterator

HashTable	HashMap	LinkedHashMap
Synchronized	Unsynchronized	It is a subclass of HashMap. It inherits all the features of HashMap. In addition linkedlist preserved the insertion order.
Doesn’t permit null key	Permits one null key