java铸造ClassCastException

Question

I have the following classes:

public abstract class A implements C {
    ...
}

public abstract class B extends A {
    method();
}

But when I try to do the following

A a = null;
A a = new A();
((B) a).method();

I receive a ClassCastException, does anyone have a solution?

Answer 1

B extends A, but A doesnt extend B.

everything A has B has, but A cant do the stuff declared in B

Answer 2

An instance of B is an instance of A , but an instance of A is not necessarily a B . Similar to saying "all circles are shapes, but not all shapes are circles".

You can't do what you are trying to do. That's why you get the exception.

Answer 3

I think what you want here is a constructor to take and A object and make a larger B object out of it which has the included method (which A does not have). But that doesn't make a whole lot of sense. Cant think for the life of me why you would want to do this in practice, and wouldn't just build the B object to start with.

public abstract class A implements C {
    ...
}

public abstract class B extends A {

    public B(A child){
        //Construct a B out of A here
    }

    public void method();
}

Then

 A a = null;
 A a = new A();
 new B(a).method();