subclass sub = new superclass() ??

[jaspreet atwal]

This maybe a very basic concept, but I am afraid I fail to understand this. Please help me clear the following query:
Why cannot we assign an object of Superclass to a subclass? Shouldn't this work as subclass inherits all attributes of a superclass?

[Ernest Friedman-Hill]

class Super { int x; }
class Sub { int y; }

// This isn't allowed, but let's pretend
Sub sub = new Super();

// This is why it's not allowed: we're trying to access the field y
// of an instance of the class "super". This would compile, but fail
// when the program ran.
sub.y = 3;

// But on the other hand, this is legal:
Super sup = new Sub();

// And this is fine because a Sub has an x, since it's
// a subclass of Super.
sup.x = 3;

[Jan van Mansum]

I suppose you meant to say that

    
class Sub extends Super { int y;}

[Jesper de Jong]

Originally posted by jaspreet atwal:
Why cannot we assign an object of Superclass to a subclass? Shouldn't this work as subclass inherits all attributes of a superclass?

There is an "is a" relationship between subclasses and superclasses. An instance of a subclass is an instance of the superclass. That is why you can assign an instance of a subclass to a variable of the superclass type - not the other way around!

class Animal {
  // ...
}
 
class Cow extends Animal {
  // ...
}
 
// Legal, because a Cow is an Animal.
Animal a = new Cow();
 
// Illegal, because it doesn't work the other way around:
// An animal is not always a cow!
Cow c = new Animal();

[ November 06, 2007: Message edited by: Jesper Young ]

[Ernest Friedman-Hill]

Originally posted by Jan van Mansum:
I suppose you meant to say that

    
class Sub extends Super { int y;}

Yes, thank you.

[jaspreet atwal]

Thank you so much for your replies! I understand a lot better now.