Java Casting Interface to Class

Question

public class InterfaceCasting {

    private static class A{}

    public static void main(String[] args) {
        A a = new A();
        Serializable serializable = new Serializable(){};
        a = (A)serializable;
    }

}

Compilation succeed but Runtime exception

Exception in thread "main" java.lang.ClassCastException: InterfaceCasting$1 cannot be cast to InterfaceCasting$A

WHY COMPILATION SUCCEED? Compiler must known that serialiazable is not A?

Answer 1

As you point out, this will compile:

interface MyInterface {}

class A {}

public class InterfaceCasting {
    public static void main(String[] args) {
        MyInterface myObject = new MyInterface() {};
        A a = (A) myObject;
    }
}

This however, will not compile:

interface MyInterface {}

class A {}

public class InterfaceCasting {
    public static void main(String[] args) {
        A a = (A) new MyInterface() {}; // javac says: "inconvertible types!"
    }
}

So, what's going on here? What's the difference?

Well, since MyInterface is simply an interface, it could very well be implemented by a class that extends A, in which case the cast from MyInterface to A would be legal.

---

This code for instance, will succeed in 50% of all executions, and illustrates that the compiler would need to solve possibly undecidable problems in order to always "detect" illegal casts at compile time.

interface MyInterface {}

class A {}

class B extends A implements MyInterface {}

public class InterfaceCasting {
    public static void main(String[] args) {
        MyInterface myObject = new MyInterface() {};
        if (java.lang.Math.random() > 0.5)
            myObject = new B();
        A a = (A) myObject;
    }
}