Optional method in Collection interface [duplicate]

Question

This question already has answers here:

Optional Methods in Java Interface (13 answers)

Closed 8 years ago.

What does "optional" mean in this context?

public interface Collection<E> extends Iterable<E> 
{
 // Basic operations
   int size();
   boolean isEmpty();
   boolean contains(Object element);
 // optional

The last line comment "//optional": what does it mean? I tried to implement a Collection by implementing it public class Col-implementation implements Collection and it's not optional at all.

I need to implement all the methods, even the ones commented as "optional". I am clearly not getting what they mean by "optional". Any insight about that?

Answer 1

You must provide an implementation, but that implementation can throw an UnsupportedOperationException.

Answer 2

In this context optional means that not all Collection<E> may do something useful in the implementation. Although providing an implementation is indeed mandatory, it is OK to throw an exception to indicate that the method is not implemented.