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When we have use some personaised object (not String,Integer etc which already implements these functions)as a key in Hash related data structures i.e. HashMap,TreeMap, HashSet etc then for correct implementation of this key, We need to implement these functions.
It is good practice to implement both these functions together.

This will the correctness of the code - For example if u r using Hashmap it uses equals and hashcode if u override one and not the other then Hashmap will be inconsistent.

you need to override the hashcode method if you override the equals method as the equals method uses the hashcode method to compare two objects of the specified type.