Java类扩展的功能

Question

Assume the following classes exist:

public class A {

  public void whatAreYou() {
    System.out.println("A");
  }
}

___________________________

public class B extends A {

  public void whatAreYou() {
    System.out.println("B");
  }
}

Now, we initialize a and b the following way:

A b = new B();
b.whatAreYou();

_______________

B a = new A();
a.whatAreYou();

The question is: Do these programs work without any mistakes? If not, what can't of mistakes do they have?

My opinion:

I think the first program should work fine since the class B is an extension of the class A. But I can't tell what kind of object we have here. Does it possess every attribute of B too? Besides that, it should print "B".

The second program does not work, but I lack a clear explanation.

Can somebody help me out?

Answer 1

Placing your code in an IDE will surely help, as you can use a debugger to see what's happening step by step. You'll also be able to see the compile time error for 'B a = new A();' and go from there. Regarding the functionality of the classes, in this case B is Overriding the 'whatAreYou()' method. This can be made clear with an @Override annotation.