继承类的Classcast异常

Question

I have a class structure like this:

Interface A extends X
Class A_Impl implements A

Interface B extends A
Class B_Impl extends A_Impl implements B

My webservice client returns object of A and I need some parameters from B. So I do is this:

A myA = (A) webservice.getA();
B myB = (B) myA;

But this always throws the ClassCast exception:

java.lang.ClassCastException: A_Impl cannot be cast to B

Am I doing something wrong here ? How can I get some params from B class.

Answer 1

You cannot get any properties of B when you have an instance of B 's supertype A , or any subtype of that other than B . For example, if you wanted to get the value of a field x that is a member of B , but your object is only an A , the field is not even present in the object. So what would the value of it be? That is the reason you can't cast in this direction. If you wan't to access the object like an instance of B , you have to change your webservice.getA() to something that actually returns a B (or a B_Impl )

Answer 2

If you have a reference to an object that doesn't implement B , there is no way to cast it to a B . Full stop.

Imagine if it was possible. Then what would this print?

interface A {
    int getNumberOfLives();
}

interface B extends A {
    boolean isOrange();
}

class A_Impl implements A {
    int getNumberOfLives() {return 9;}
}

public class Main {
    public static void main(String[] args) {
        A a = getA();
        B b = (B)a;
        System.out.println(b.isOrange() ? "Is orange" : "Is not orange");
    }

    static A getA() {return new A_Impl();}
}