Cast with Runnable interface.

[Adolfo Eloy]

Hello all.

I'd like to understand why line1 compiles and line 2 doesn't.
When I remove the final modifier from D class, line 2 compiles.

Based on an exercise from ExamLab (Practice exam 2)

class A {}
class B extends A {}
class C extends A {}
final class D extends B {
	public void doMethod() {
		A a = new B();
		B b = new D();
		C c = new C();
		D d = null;
		Runnable r1=(Runnable)c; // LINE 1 - COMPILES
		Runnable r2=(Runnable)d; // LINE 2 - INCONVERTIBLE TYPES
	}	
}

Thanks

[Henry Wong]

Adolfo Eloy wrote:

		Runnable r1=(Runnable)c; // LINE 1 - COMPILES

In this case, the compiler can't tell if the C instance, is just a C instance, or some subclass of C (which IS-A C) that also happens to implement the Runnable interface.

Adolfo Eloy wrote:

		Runnable r2=(Runnable)d; // LINE 2 - INCONVERTIBLE TYPES

In this case, the compiler can tell. Since the D class is final, the D instance is just a D instance. And since the D class doesn't implement Runnable, the cast is not allowed.

Henry