Question

As we know, interfaces can extend interface in the Java. I have a question about this, if interface B extends interface A, B need not implement the methods defined in the A. But in the java.util package, the List interface extends Collection interface, and it implements the Collection method, these methods also just have the method declaration.

Why does it do this, and it there a better practice? Does it make any difference between implementing the method in the sub interface or not?

Answer 1

Overriding a method, besides providing/replacing a method implementation, allows to provide a more specific javadoc, and to narrow the return type.

For instance, Collection.iterator() is specified by:

Returns an iterator over the elements in this collection. There are no
guarantees concerning the order in which the elements are returned
(unless this collection is an instance of some class that provides a
guarantee).

while List.iterator() is specified by

Returns an iterator over the elements in this list in proper sequence.