Interfaces

Question

Why should I create interfaces in java?
Can I describe functions of interface in one class and then use them just by implementing the Interface and call them or I should describe function in every class I implementing the Interface?
As i know KeyListener is an Interface. How can I make something similar to KeyListener with interfaces functions that are called automatically?

Answer 1

Like KeyListener I mean to be called automatically when something happens.
For example when something is moved then check for collision or something like that!

Answer 2

This question requests lines in order, but the main class calls lines in wrong order. Is this intentional (so we find the error to pass) or is there something I'm missing to make the lines come out in right order without changing the main class?