javaclasscastexceptionextends

Downcast gives ClassCastException

Two classes:

public class ClassA implements ClassC {
    private static final long serialVersionUID = 1111111111111111111L;
    public String varA;
}

public class ClassB extends ClassA implements ClassC {
    private static final long serialVersionUID = 4604935970051141456L;
    public String varB;
}

In my code I have this declaration: ClassB classB;

and a method

myMethod()

that returns a ClassA object.

If I write:

classB = (ClassB) myMethod();

I thought that I will have:

classB.varA = <a value from myMethod()>
classB.varB = null

but this is not so: I receive a ClassCastException.

Why? And how can assign the myMethod() to ClassB in a fast way?

Solution

  • The problem is that you're trying to downcast a ClassA to a ClassB, which isn't allowed because the actual object might be ClassX (another subclass of ClassA) which doesn't have an explicit conversion.

    See the following example which will throw a ClassCastException (but compiles just fine):

    public class Main {
    public static void main(String[] args) {
        SubB var = (SubB) someMethod();         
    }

    private static Super someMethod(){
        return new SubA();
    }
}

class Super { }
class SubA extends Super { }
class SubB extends Super { }

    The code compiles perfectly:

    Yet when you execute this, you'll try to cast an object of type SubA to SubB, which isn't possible.

    Sidenotes: