Java casting in interfaces

casting, interface, java

Solution

When you cast `o1` and `o3` with `(I2)`, you tell the compiler that the class of the object is actually a subclass of its declared type, and that this subclass implements `I2`.

The `Integer` class is final, so `o3` cannot be an instance of a subclass of `Integer`: the compiler knows that you're lying. `C1` however is not final, so `o1` could be an instance of a subtype of `C1` that implements `I2`.

If you make `C1` final, the compiler will complain too:

interface I1 { }
interface I2 { }
final class C1 implements I1 { }
class C2 implements I2 { }

public class Test{
     public static void main(){
        C1 o1 = new C1();
        C2 o2 = new C2();
        Integer o3 = new Integer(4);

        I2 y = (I2)o3; //compiler complains here !!
        I2 x = (I2)o1; //compiler complains too
     }
}

Problem

Original source