Use of hashCode() and equals(). 

Object class provides two methods hashcode() and equals() to represent the identity of an object. It is a common convention that if one method is overridden then other should also be implemented.

Before explaining why, let see what the contract these two methods hold. As per the Java API documentation:

• Whenever it is invoked on the same object more than once during an execution of a Java application, the hashcode() method must consistently return the same integer, provided no information used in equals() comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
• If two objects are equal according to the equals(object) method, then calling the hashCode() method on each of the two objects must produce the same integer result.
• It is NOT required that if two objects are…