Question

Very simple code for test:

interface Base {
    void interfaceTest();
    static final String m = "1";
}

interface Child extends Base {
    void interfaceTestChild();
}

class BaseClass implements Base {
    @Override
    public void interfaceTest() {
        System.out.println("BaseClassInterfaceTest");
    }
}

class ChildClass implements Child {

    @Override
    public void interfaceTest() {
        System.out.println("ChildClassInterfaceTest");
    }

    @Override
    public void interfaceTestChild() {
        System.out.println("interfaceTestChild");

    }
}

public class Src {
    public Child testFunc() {
        Base x = new BaseClass();
        return (Child)x;      <==Here got an "ClassCastException"
    }

    public static void main(String args[]) {
        Src testSrcInstance = new Src();
        testSrcInstance.testFunc().interfaceTest();
    }
}

In the line return (Child)x; I got a “ClassCastException” and I feel very confused about it, for Child extends Base, so x should be converted to Child successfully. this kind of conversation is imitated by some android codes:

the getText() method of EditText is:

public Editable getText() {
        return (Editable) super.getText();
    }

and the super class of EditText is TextView, of which the getText() method is:

public CharSequence getText() {
    return mText;
}

mText is a CharSequence, and note that Editable extends CharSequence, so you can see, these android codes cast CharSequence to Editable, just as me, cast Base to Child, any difference?

Answer 1

in the line return (Child)x; i got a “ClassCastException”, i feel very confused about it, for Child extends Base, so x should be converted to Child successfully.

No, it will only work if x *actually refers to an instance of some type which implements Child. In this case, it doesn’t – it only refers to an instance of BaseClass. That doesn’t specify any behaviour for interfaceTestChild(), so what would expect to happen if you’d been able to call it?

Base x = new BaseClass();
// Imagine this had worked...
Child child = (Child)x;
// What would this do? There's no implementation!
child.interfaceTestChild();

Java only lets you cast to a type which the value actually supports – i.e. some type in the inheritance hierarchy of the object that the value refers to.