what exactly overriding is?

[Arvind Giri]

hi friends,
please tell me what exactly overriding is?

[Barry Gaunt]

You did say exactly? Then take this: Java Language Specification on method overriding

[swathi mims]

overriding is what happens to your methods when u do inheritence.lets for example take CLASS A and CLASS B WHICH EXTENDS CLASS A.assume class A has a method named public void Hello() and class B too has the same class with the same name and same arguements and return type.Then it tries to override the method from its superclass.when you thus try to access the hello() from main it will first access the overridden method from class B unless ther is a reference variable specified that refers to its superclass.

Hope u got it

[Barry Gaunt]

Note that in the JLS definition there is the concept of subsignature. This means that the superclass method could have a parameter of type List<? extends Number> for example, and the overriding method could have List (just List, the raw type) as its parameter type.

Also for reference return types, the overriding method's return type can be a subclass of the overridden method's return type.
[ September 11, 2005: Message edited by: Barry Gaunt ]

[Arvind Giri]

Hi Berry,
That link is really useful.
However I have doubt about the overriding method's return type can be a subclass of the overridden method's return type. as following code is not working.
plz clear my doubt

class A
{
    public A returnObject()
{
 return new A();
}
}
 
class B extends A
{
   
   public B returnObject()
{
 return new B();
}
}  

thanks in advance

[Sandeep Chhabra]

Originally posted by Arvind Giri:
...as following code is not working.
plz clear my doubt ...

class A
{
    public A returnObject()
{
 return new A();
}
}
 
class B extends A
{
   
   public B returnObject()
{
 return new B();
}
}  

thanks in advance

Hi Arvind,
well the code is compiling and running with no errors
here is the modified version:

 
class A
{
    public A returnObject()
{
 return new A();
}
}
 
class B extends A
{
   
   public B returnObject()
{
 return new B();
}
}  
 
public class test26
{
A a=new A();
B b=new B();
A x=a.returnObject();
B y=(B)a.returnObject();//cast needed
}
 

here "B y=(B)a.returnObject();//cast needed" cast is required since the overriden method returns the object of class A.

Please correct me if i am wrong...

[Barry Gaunt]

You code compiles OK for me: Are you using Java 5.0?
With Java 5.0 this code for class B:

class B extends A
{
   
   public B returnObject()
{
 return new B();
}

   public static void main(String[] args) {
       A b = new B();
       System.out.println(b.returnObject() instanceof B);
    }
}    

runs as expected printing true showing that method returnObject() is being properly overridden.

[Arvind Giri]

hi friends,
I am still feeling problem in compiling that code

class A
{
    public A returnObject()
{
 return new A();
}
}
 
class B extends A
{
   
   public B returnObject()
{
 return new B();
}
}  
 
public class test26
{
A a=new A();
B b=new B();
A x=a.returnObject();
B y=(B)a.returnObject();//cast needed
}
  

when I compiled above code I got the following error.

C:\j2sdk1.4.2_09\bin>javac test26.java
test26.java:12: returnObject() in B cannot override returnObject() in A; attempt
ing to use incompatible return type
found : B
required: A
public B returnObject()
^
1 error

Plz tell me where I am wrong?

[Barry Gaunt]

You are using a version 1.4.2 compiler on code meant for the version 5.0 compiler.

[Arvind Giri]

hi Berry,
thanx for making it clear to me.