Method signature and method overriding.

[dmjpro]

*************** ***********
Sorry the title would be Overriding. Please experts, correct the title ;)
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Method signature includes ...
1.Modifiers
2.Return type
3.Method name
4.Number of passing parameters and types of parameters
5.Throws clause

No doubt method name and number of passing parameters must be same.

Return type must be same or it may be the super class type in super class and sub class type in subclass.
Have a look ...

class AClass{
 public Number test(){}
}
 
class BClass extends AClass{
 @Override
 public Integer test(){}
}
 

Now come to the throws clause. It is similar as return type.

class AClass{
 public void test()throws Exception{}
}
 
class BClass extends AClass{
 @Override
 public void test()throws IoException{}
}
 

Now come to the types of passing parameters.
Have a look at this..

class AClass{
 public void test(Interger i){}
}
 
class BClass extends AClass{
 @Override
 public void test(Number num){}
}
 

Here it says that method test doesn't get overridden :(
I think here is no chance of ClassCastExcept ion.

AClass a = new BClass();
a.test(new Integer(100)); //here is no chance of ClassCastException
 

Then why it flashes an error?

[r035198x]

The reason is what you said above that
"Return type must be same or it may be the super class type in super class and sub class type in subclass." You have it the other way round in that class so it won't compile.
You also left out the order of the passing parameters. The order has to match for an override to take place.

P.S Your typing is still bad. Integer != Interger. Better copy and paste the code straight from your editor.

[dmjpro]

@r035198x
I couldn't get your point here. If Java allowed it then what would happen?

@r035198x
Oh yeah, i left that ;)

@r035198x
Actually i simply wrote it. Sorry again!

By the way why didn't you change the title? I think you got the privilege ;)

[r035198x]

@dmjpro
Then it wouldn't be overriding because your point #(2) fails. The return types must match or be covariant for a true override.

[dmjpro]

@r035198x
You are talking about return type. But i was talking about passing parameters.
Still i am not getting the reason.

[r035198x]

Ok on the third snippet it's because the parameter Number can't be used in place of an Integer.
Integer i = 9;
Number n = i;works
But i =n does not work.

[dmjpro]

@r035198x
How i=n comes here. In the 3rd code snippet it will be n=i, and it works fine.

[JosAH]

@dmjpro
Overriding is not a conditional thing where the coice is made based upon the type of the parameter(s); that's a job for the overloading mechanism which the compiler can handle; your annotation is wrong there: remove it and the compiler will happiliy create an overloaded method for you.

kind regards,

Jos

[dmjpro]

Well.....
The return type can be co-variant. But in case of parameter(s) type what's wrong with that?

[dmjpro]

Well ...
Now i can figure out ...

class AClass{
    public Number test(Integer i)throws Exception{
        System.out.println("In Super Class");
        return null;
    }
}
 
class BClass extends AClass{
    //@Override
    public Integer test(Number num)throws IOException{
        System.out.println("In Derived Class");
        return null;
    }
 
    @Override
    public Integer test(Integer i)throws IOException{
        System.out.println("In Derived Class");
        return null;
    }
}