Optional Methods in Java Interface

Question

From my understanding if you implement an interface in java, the methods specified in that interface have to be used by the sub classes implementing the said interface.

Answer 1

If we go through the code of AbstractCollection.java in grepCode which is a ancestor class for all collection implementations, it will help us to understand the meaning of optional methods. Here is the code for add(e) method in AbstractCollection class. add(e) method is optional according to collection interface

public boolean  add(E e) {

        throw new UnsupportedOperationException();
    } 

Optional method means it is already implemented in ancestor classes and it throws UnsupportedOperationException upon invocation. If we want to make our collection modifiable then we should override the optional methods in collection interface.