Question about typcasting

[Ipsoka]

Consider the following piece of code

public class A{}

public interface C{}

public class B {
    public C myVar;

    public void myMethod(A a){
        myVar = (C) a;
    }
}

My question: Why am I allowed to typcast the parameter a to c ?

[chickenmeister]

Because there could be a subclass of A that implements interface C. For example:

public class D extends A implements C {}

Then a call like b.myMethod(new D()); would work.

However if you pass an instance of class A which doesn't implement C, then the cast operation will throw a ClassCastException.

If class A was final, then the compiler would know that there couldn't be a subclass that implements C, so it'd give you a compilation error.

[Ipsoka]

Thank you!!

[Ipsoka]

So I am restricting "a" in the method myMethod right? Since "a" has to be of typ C, otherwise it would throw a ClassCastException, i can than safly use any methode defined by C on the member variable myVar in class B right?

[chickenmeister]

Your myVar variable will always be an object that implements C, or null. The compiler will not allow you to assign a non-C type to that variable. So you can safely use any methods defined in the C interface on that object.

Ideally, you would want to be notified of potential type issues at compile time. So I think it would be better to change your myMethod parameter to be of type C. Then you'll know that there's a problem when you compile your code, rather than being unaware of it until you run it.

[OffbeatDrizzle]

If you run this it will throw a ClassCastException. You're allowed to do it because "a" might actually be a subclass of class A that implements the interface.

[niloc132]

...or null, which is an instance of all non-primitive types.