Question

Say i have an interface I with method doIt();

Now i have a class C that implements I and hence provides the implementation for doIt();

For the following code in Class Hello

public class Hello{
    I i = new I();
    i.doIt();
}

1.) Now where does it take the implementation of doIt() from?

2.) What if there was another class HelloWorld implementing I too with a different iplementation for the doIt() function. Then which implementation would it have taken?

Answer 1

Try compiling this stuff. You can only instantiate a Class which implements the Interface and not the Interface directly.

An Interface is just like a contract which a Class agrees to when it implements it; and within the agreement of the Interface, Class provides the implementation of it.