Invoking superclass methods... how?

[rhobincu]

Hi,

I'm not really new to Java but I'm posting here since the problem appears to simple for the advanced forum. I've written these 2 test classes:

Code:

public class A{
    public void doS(){
        System.out.println("A.doS()");
    }
}

and

Code:

public class B extends A{
    public void doS(){
        System.out.println("B.doS()");
    }

    public static void main(String args[]){
        B b = new B();
        b.doS();
        ((A)b).doS();
    }
}

Now, I was under the impression that since I casted the "b" object to class "A", invoking the "doS()" method will call the corresponding method from class "A". However the result is:

Code:

[rhobincu@st27 stuff]$ java B
B.doS()
B.doS()

Disassembling the code made it even weirder:

Code:

[rhobincu@st27 stuff]$ javap -c B
Compiled from "B.java"
public class B extends A{
public B();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method A."<init>":()V
   4:   return

public void doS();
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #3; //String B.doS()
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

public static void main(java.lang.String[]);
  Code:
   0:   new     #5; //class B
   3:   dup
   4:   invokespecial   #6; //Method "<init>":()V
   7:   astore_1
   8:   aload_1
   9:   invokevirtual   #7; //Method doS:()V
   12:  aload_1
   13:  invokevirtual   #8; //Method A.doS:()V
   16:  return

}

I'm noticing that the second invokevirtual instruction really calls A.doS()V with no apparent success. I also notice no "checkcast" instruction.

Is there a problem with my JDK? Or am I missing smth?

Appreciating any suggestions,
Radu

[henry_78]

Hi,
The output of your code is ok.
When u invocke a method on an object, at runtime the virual machine is able to know the specific class to which your object belongs.
In your example code, u create an object which belongs to the class B, even if u cast it, when u call a method that u have overwritten, at runtime will be executed the method of the specific class of the object( the is the method in the class B).
Bye.

[rhobincu]

Ok, so the only way to invoke an overwritten superclass method is to write a method in your extended class that does "super.doS()"?

What if I have:

java.lang.Object ---> testClass.A ---> testClass.B ---> myTestClasses.C

If all A, B and C classes have a method called "doS" there is no way to access testClass.A.doS() from myTestClasses.C considering I only have the compiled code (.class / .jar files) for classes A and B?

[henry_78]

Yes, there will be no way...if u really need it u can just not overwrite it.
U dont have the source code????

[rhobincu]

It's not that I don't have the source code, I'm just trying to figure out EXACTLY how JVM works becouse we're working at a Java hardware machine and we need to get it right.

At the first sight it seemed normal to be able to do what I wrote in the first post but it seems that I was wrong. I'm wondering how many surprises I'm up for.

Thanks for your help tho,
Radu

[henry_78]

Regarding polimorfism,
U can have many superclasses as u like till u arrive to the Object class that is the superclass of all classes. There is no limit to the superclsses.

[rhobincu]

Yes, I understand that, it was the issue of accessing the public & protected fields & methods in those superclasses that bugged me. Especially if some of those classes overwrite methods.

[henry_78]

Hope u got things clear now..if u dont u can just ask whatever u want,
Bye.