What is the need to override the equals and hashCode methods in Java

Question

How can I decide when to use equals() and hashCode() efficiently?

Answer 1

You must override hashCode() in every class that overrides equals(). Failure to do so will result in a violation of the general contract for Object.hashCode(), which will prevent your class from functioning properly in conjunction with all hash-based collections, including HashMap, HashSet, and Hashtable.

- from Effective Java, by Joshua Bloch

By defining equals() and hashCode(), you can improve the usability of your classes as keys in hash-based collections. As the API doc for hashCode explains: "This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable."