HashMap is well known class and all of us know that. So, I will leave this part by saying that it is used to store key-value pairs and allows to perform many operations on such collection of pairs.

TreeMap is special form of HashMap. It MAINTAINS the ordering of keys which is missing in HashMap class. This ordering isby default “NATURAL ordering”. The default ordering can be override by PROVIDING an instance of Comparator class, whose COMPARE method will be used to maintain ordering of keys.

Please note that all keys inserted into the map must implement the Comparable interface (this is necessary to decide the ordering). Furthermore, all such keys must be mutually comparable: k1.compareTo(k2) must not throw a ClassCastException for any keys k1 and k2 in the map. If the USER attempts to put a key into the map that violates this constraint (for example, the user attempts to put a string key into a map whose keys are integers), the put(Object key, Object value) call will throw a ClassCastException

HashMap is well known class and all of us know that. So, I will leave this part by saying that it is used to store key-value pairs and allows to perform many operations on such collection of pairs.

TreeMap is special form of HashMap. It maintains the ordering of keys which is missing in HashMap class. This ordering isby default “natural ordering”. The default ordering can be override by providing an instance of Comparator class, whose compare method will be used to maintain ordering of keys.

Please note that all keys inserted into the map must implement the Comparable interface (this is necessary to decide the ordering). Furthermore, all such keys must be mutually comparable: k1.compareTo(k2) must not throw a ClassCastException for any keys k1 and k2 in the map. If the user attempts to put a key into the map that violates this constraint (for example, the user attempts to put a string key into a map whose keys are integers), the put(Object key, Object value) call will throw a ClassCastException