为什么以下代码会给出类广播异常？

Question

Given below piece of code:

//Super class 
class A {
    void m1() {
        System.out.println("A");
    }
}

//Extending the super class  
class B extends A {
    void m1() {
        System.out.println("B");
    }
}

// Class with the main method
public class C extends B {
    void m1() {
        System.out.println("C");
    }
}

//in some main method
B b1 = new B(); //creating B object
System.out.println(b1 instanceof A); //gives true
B b = (B) new A();
b.m1();

1. The instanceof operator gives true so B is a object of class A .
2. But why while casting A to B its gives ClassCastException even though B is an sub-type of A ?

Answer 1

new A(); is simply not B .

It's the same as you were saying that "Every Animal is a Cat with no exceptions". Which is not true.

However, casting B to A would be correct (because every Cat is an Animal, with no exceptions):

A a = (A) new B();

Answer 2

A a = new B(); // is fine as B inherits A
B b = new A(); // is NOT fine as A does not inherit any thing from B

Answer 3

B is an instance of A that is right. But A is not an instance of B. That's why you get the exception.

Answer 4

B b = (B) new A(); is the wrong statement cause A doesn't inherits B ; rather B inherits A

Answer 5

Allowing such casting would not be safe for few reasons.

1) Lets say that you added in your B additional method of field which A doesn't have like

class A {
    void m1() {
        System.out.println("A");
    }
}

class B extends A {
    void m1() {
        System.out.println("B");
    }

    private int x;
    public void setX(int x) {
        this.x = x;
    }
}

Now lets assume that this casting will work

B b = (B) new A();

So b will hold instance of A . But later we can try invoking setX from B reference

b.setX(1);

which should be fine because there was no casting exception so object from B reference should be able to support this call.
What do you think, how should JVM react on this next line? Should such code be allowed?

No, base A doesn't know how which code should be executed for setX and even if it knew it, it doesn't have int x field to store its data. So to prevent such situations JVM throws ClassCastException.

---

2) You can say that: " OK, but my B doesn't change interface, it doesn't add anything new to A ". You are right, but allowing casting in this case is also dangerous, because while invoked methods from B you should be able to expect that they will behave in a way described in B class, or its subclasses.

Lets say that we are building racing game and we have classes

class Car{
    int getMaxSpeed(){
        200;
    }
}
class LimitedSpeedCar extends Car{
    int getMaxSpeed(){
        return 30;
    }
}

Now you want to create racing track only for cars with limited speed. So you are creating LimitedSpeedCar[] array to store only cars with limited speed. But guess what, because

B b = (B) new A();

works fine it means that

LimitedSpeedCar lsc = (LimitedSpeedCar) new Car();

will also work so we can add to our race car without limits which is not what we wanted.

Answer 6

B is a subclass of A , you cannot cast A to B . In fact, maybe the concrete class of the new A Object is another class than B (let say a class C that extends A ). So you cannot affirm that A is an instance of B .

Answer 7

An object of type B can be cast to variable of type A but not the other way around. B extends A so it could potentially have more functionality. This might be a good time to go back and read up on polymorphism.

Answer 8

You can't down cast an object. You can create a

A a1 = new B() .

But you can't create a

B b1 = new A()

It's simple like every German Shepard is a Dog. So you can create a new German Shepard dog.

But you can't be sure that every Dog is a German Shepard. Hence creating a new German Shepard of type dog is not supported. Read This