question about declare and create an object

[VuTuan Tran]

There are 2 classes and one is subclass of another like below:

class A { }
class B extends A {
1. A x = new B();  // x is a *reference variable* of class type A and it points to( refer to) an *object* of class type B
2. B x = new A();  // x is a *reference variable* of class type B and it points to( refer to) an *object* of class type A
}  

The first one is correct one but the second. I do not have a good explanation for that. I do know that

A reference variable can refer to any object of its declared type or any subtype of its declared type

and why not

A reference variable can refer to any object of its declared type or any supertype of its declared type

Im not convinced myself. Please help, thanks

[swaraj gupta]

"A super class reference variable can refer to any direct or indirect sub class object, but it reverse is not allowed."

And the reverse is not allowed becuse it is possible that super class might have members (privete) which are not available in sub class.

[Rob Spoor]

Try replacing A and B with more concrete examples:

class Animal {}
class Dog extends Animal {}

Animal x = new Dog(); // because every Dog is an Animal
Dog x = new Animal(); // not every Animal is a Dog; what about Cats? Fish? Birds?

[Campbell Ritchie]

swaraj gupta wrote:"A super class reference variable can refer to any direct or indirect sub class object, but it reverse is not allowed."

That is correct, but please tell us where you found it, to avoid copyright problems, etc.

And the reverse is not allowed becuse it is possible that super class might have members (privete) which are not available in sub class.

That doesn't seem a very good explanation; looking at Rob's post with Dog and Animal gives an easier explanation to understand.