please help out about object reference...

[shankar dengi]

class Display
{
void display()
{
System.out.println("hi");
}
}
class Disp1 extends Display
{
void disp1()
{
System.out.println("hi1");
}
}
class Dem

{
public static void main(String [] arg)
{
Display b=new Disp1();//this compile fine
Disp1 a=new Display();//y dis wont compile?

}
}
in above "b" reference know about only members of Display in Disp1 object so it compiles fine even reference "a" know about both den why it give compile error?
please help me...

[Henry Wong]

shankar dengi wrote:
Display b=new Disp1();//this compile fine
Disp1 a=new Display();//y dis wont compile?

A Disp1 object IS-A Display. The reverse is not necessary true. And in this example, is definitely not true.

Henry

[shankar dengi]

Henry Wong wrote:

shankar dengi wrote:
Display b=new Disp1();//this compile fine
Disp1 a=new Display();//y dis wont compile?

A Disp1 object IS-A Display. The reverse is not necessary true. And in this example, is definitely not true.

Henry

I dint understood please explain me in detail..

[Rob Spoor]

Let's replace Display and Disp1 with Animal and Cat.

Animal b = new Cat(); // sure, because every cat is an animal
Cat a = new Animal(); // uh oh... not every animal is a cat. What about dogs, horses, birds?

[shankar dengi]

Rob Prime wrote:Let's replace Display and Disp1 with Animal and Cat.

Animal b = new Cat(); // sure, because every cat is an animal
Cat a = new Animal(); // uh oh... not every animal is a cat. What about dogs, horses, birds?

ya but how it works internally?

[Tom Reilly]

From a practical sense, if the compiler allowed you to assign a reference to a Display instance to a Disp1 reference variable as in:

Disp1 a=new Display();//y dis wont compile? 

you could call the method a.disp1(), which is in the Disp1 class - not the Display class. But the instance you created is a Display. So at runtime, the JVM would at best throw an exception and at worst, crash.