extending superclass and ClassCastException

问题

I have a superclass, which two methods i want to override. Here's my code:

public class MyCustomClass extends SomeSuperClass {

protected MyCustomClass(params) {
    super(params);
}
@Override
public void method1() {
    super.method1();
    /* here goes my code */
}
@Override
public void method2() {
    super.method2();
    /* here goes my another code  */
}

I have some constructor, that passes SomeSuperClass object as a parameter, and what i do next:

MyCustomClass object;
/* now i have object of type SomeSuperClass,
but with my own method1() and method2() */
object = (MyCustomClass) MyCustomClass.item(blahblah); 
/* eclipse suggests casting, because MyCustomClass.item()
 constructor still returns SomeSuperClass object */
otherobject = OtherConstructor.object(object);
//OtherConstructor passes SomeSuperClass object

That seems to be right, but i'm getting java.lang.ClassCastException in SomeSuperClass while executing.

if i create SomeSuperClassObject, i lose my overriden methods.

With casting, even if there's no errors in eclipse, application crashes. In other words, how i can override SomeSuperClass with my own methods, and still get SomeSuperClass object to use with OtherConstructor? If it is important, this code is for android app.

回答1:

As a general rule, you can cast an instance of a subclass to its parent class:

MyCustomClass object = new MyCustomClass(params);
SomeSuperClass superClass = (SomeSuperClass) object;

However, you cannot cast an instance of a superclass to a subclass:

SomeSuperClass object = new SomeSuperClass(params);
MyCustomClass customClass = (MyCustomClass) object; // throws ClassCastException

This is because a MyCustomClass object is also a SomeSuperClass object, but not all SomeSuperClass objects are MyCustomClass objects.

You may be able to work around this with certain design patterns. Java itself tends to use the Decorator pattern a lot.

回答2:

From what I see it seems that MyCustomClass.item(blahblah) call returns something different (maybe the parent) than MyCustomClass. Its the only part in te code, where you are casting object...

回答3:

If the item() method is declared in SomeSuperClass, I doubt that it is returning an instance of MyCustomClass. So your cast (MyCustomClass) MyCustomClass.item(blahblah) would be invalid.