doubt in inheritance

[arulraj michaelraj]

i am arulraj and have a doubt in inheritance..

please see the following example.

class A
{
int a=10;
}
class B
{
int a=20;
}

A objA=new A();
B objb=new B();

objA=objb; // 1
objb=objA // 2 it says compiler an error.


In the above example , if you assign a subclass object to a superclass object, the program compiles and executes...

if you do reverse operation (assign a superclass object to subclass object), the compiler says an error....

1. Can you pleas explain it why this one is not allowed?

2.
System.out.println("objA.a:"+objA.a);
it prints value 10.
May i know why it gives the value 10 instead of 20?

3.. what is difference between object and object reference?

[marc weber]

"arulrajjj michaelrajjj,"

Welcome to JavaRanch!

Please revise your display name to meet the JavaRanch Naming Policy. To maintain the friendly atmosphere here at the ranch, we like folks to use real (or at least real-looking) names.

You can edit your display name here. Thank you for your prompt attention!

-Marc

[arulraj michaelraj]

i am arulraj and have a doubt in inheritance..

please see the following example.

class A
{
int a=10;
}
class B extends A
{
int a=20;
}

A objA=new A();
B objb=new B();

objA=objb; // 1
objb=objA // 2 it says compiler an error.


In the above example , if you assign a subclass object to a superclass object, the program compiles and executes...

if you do reverse operation (assign a superclass object to subclass object), the compiler says an error....

1. Can you pleas explain it why this one is not allowed?

2.
System.out.println("objA.a:"+objA.a);
it prints value 10.
May i know why it gives the value 10 instead of 20?

3.. what is difference between object and object reference?

[Jesper de Jong]

Inheritance indicates an "is a" relationship. An instance of a subclass is a specialized version of an instance of its superclass. It is easier to see if you use other names than "A" and "B" for your classes. For example:

class Animal {
    // ...
}
 
class Cow extends Animal {
    // ...
}
 
Animal a = new Animal();
Cow b = new Cow();
 
// This is OK because a Cow is an Animal
a = b;
 
// This is wrong because not every Animal is necessarily a Cow
b = a;

About question 2: Member variables cannot be overridden just like methods.

About question 3: An object is "the thing itself" and a reference is something that points to the object. In Java, variables are references to objects (unlike for example C++ where variables hold the objects themselves).
[ March 08, 2007: Message edited by: Jesper Young ]

[arulraj michaelraj]

thanks jesper....