Lets add some good scjp 5 questions here

[Abdul Mohsin]

Please add some good scjp 5 questions here

[Abdul Mohsin]

How many methods contains the compiled class B?

class A<T> {
T ob;
T getOb() {
return ob;
}
void setOb(T ob) {
this.ob = ob;
}
}
public class B extends A<String> {
String getOb() {
return super.getOb();
}
}

[Barry Gaunt]

What do you mean by contained? There is a load of method inherited from class Object - you include those too?

[Abdul Mohsin]

just check the compiled class B it will contain 2 methods , just check the second method sig.

[Chandra Bhatt]

class B extends the class A with parameterized type String, and overrides
the method getOb();

B has two methods, one of them is synthetic bridge method.



Regards,
cmbhatt

[Chandra Bhatt]

Quite interesting!

class A<T> {
T ob;
    T getOb() {
        return ob;
    }
   void setOb(T ob) {
        this.ob = ob;
   }
}

public class B extends A<String> {
    String getOb() {
        return super.getOb();
    }
}

We have overridden the getOb() method of the class A, that class B inherits from A<String>. What complier does is quite different; class A's getOb() type will be Object and class B parameter type is String; To handle with this covariant return compiler adds a bridge method with Object return type in the class B.

Object getOb() {
    return (String)getOb();
}

The effect of the bridge method is to convert any calls to the inherited versions of getOb() to a call to your version, thus override of the base class method effective.


Regards,
cmbhatt
[ April 27, 2007: Message edited by: Chandra Bhatt ]

[Nik Arora]

Hi chandra,
What do you mean by bridge method.Please explain.

[Chandra Bhatt]

Hi Arora,

I hope you would have idea of "covariant return" that is allowed with Java 5.0;

Overriding methods can return the subtype of the return type of the overridden method. Suppose Parent class SpecialNumber named getNumber that returns "Number"

class SpecialNumber {
   Number getNumber() {
       return new Integer(10);
   }
}

and another class MagicalNumber extends the class SpecialNumber like:

class MagicNumber extends SpecialNumber{
   Double getNumber() {
       return new Double(10.10);
   }
}

Java 5.0 or so compiler won't complain about this and compile FINE!
But to allow us the advantage of covariant return, compiler has to do one extra work. Ask what? That is the bridge method.

Parent class method returns number and child class method returns Double.
Return type mismatch; for that compiler adds a method with the same
signature and return type inside the child class.

class MagicalNumber extends SpecialNumber{
 
   Double getNumber() {
       return new Double(10.10);
   }
 
   
   /*Bridge method added by the compiler, so that when make call to
   the method of your class, the bridge method could convert that call 
   to your version (MagicalNumber class) of the method. */
 
  [B][I] Number getNumber() {
       return (Double)getNumber();
   }[/B][/I]
}

Got it?


Regards,
cmbhatt
[ April 27, 2007: Message edited by: Chandra Bhatt ]

[Nik Arora]

Yes chandra thanks for the reply and hey i am not good in generics. Can you help me?

[Chandra Bhatt]

Yes chandra thanks for the reply and hey i am not good in generics. Can you help me?

We have so many Generics expert here! You can post your questions/doubts here,
they we surely help you.

I will also try to help you!

By the way if you have some idea of covariant return type and generics,
the above post is quite cool to explain the things well.(as I think)


Regards,
cmbhatt

[Abdul Mohsin]

Hi Chandra,

Thanks for explaining it to me.

Put some good questions of scjp 5 if you have, I will really appreciate.

Abdul