Question

I have to work with an API that is accompanied by a few examples. In one of the example, an interface is being directly used to call one of the methods of that interface. However, since Interface do not contain any implementation, I would like to know: How can the methods be used in the example to accomplish anything without defining a class that implements that interface?

Or, can the interface contain complete method definition too? (which seems to be the case here)

Answer 1

No, the interface contains only method signatures. You cannot have an implementation in an interface.

What is (most likely) happening in your case is something like (pseudocode):

InterfaceA {
    methodA();
}

class A implements InterfaceA {
   methodA() // implementation
}

InterfaceA getInterface() {
   // some code which returns an object of a class which implements InterfaceA
}

Calling method:

InterfaceA in = getInterface() // you might get an instance of class A or any other class which implements InterfaceA

in.methodA(); // implementation from whatever class the method returned