How to get a base class subobject

[Stuart Ash]

Consider class B extends class A.

For any code invoked from B, use of "this" returns an object of type B, such that this.getClass() gives "B".

How can one, if at all possible, get the base class subobject, that is, .getClass() should return A.

[Michael Dunn]

is this what you mean?

class A
{
  int x = 10;
  public void printX(int x)
  {
    System.out.println(x);
  }
}
class B extends A
{
  int x = 25;
  public B()
  {
    printX(super.x);
    printX(x);
  }
  public static void main(String[] args){new B();}
}

[Stuart Ash]

More like this...

  
class A
{
  int x = 10;
  public void printX(Object x)
  {
    System.out.println(x);
  }
}
class B extends A
{
  int x = 25;
  public B()
  {
    printX(super.x); // A
    printX(x);     // B
    printX(?????); // A  --- How??????
    printX(this);  // B 
  }
}

[ November 16, 2005: Message edited by: Stuart Ash ]

[Jan Groth]

hi stuart,

since i know that it's you who normaly posts the answers, not the questions, i was warned ;-)

i discussed it with some colleagues. we'd say that it's not possible. my naive approach was to check
- if "this" holds a reference to B
- what is "super" holding then?

well, "this" is an object reference (can for example be passed into methods), while "super" is a way to reference a field in a superclass, and cannot be passed as an argument.

long talk, short sense:

it's not possible in my opinion.

curious about other answers,
jan

[Ådne Brunborg]

I'd say you have to create an instance of A

  
class A
{
  int x = 10;
  public void printX(Object x)
  {
    System.out.println(x);
  }
}
class B extends A
{
  int x = 25;
  public B()
  {
    printX(getSuperClass(this)); // A  --- How??????
    printX(this);  // B 
  }
 
  private Class getSuperClass(B b)
  {
    if (b instanceof A)
    {
      A me = new A();
      return me.getClass();
    }
    else
    {
      return this.getClass();
    }
  }
}

[Stuart Ash]

Originally posted by Jan Groth:
hi stuart,

since i know that it's you who normaly posts the answers, not the questions, i was warned ;-)

Thanks Jan, sounds like a compliment.

Originally posted by Jan Groth:


i discussed it with some colleagues. we'd say that it's not possible. my naive approach was to check
- if "this" holds a reference to B
- what is "super" holding then?

well, "this" is an object reference (can for example be passed into methods), while "super" is a way to reference a field in a superclass, and cannot be passed as an argument.

long talk, short sense:

it's not possible in my opinion.

curious about other answers,
jan

I tried all ways too, and came to the same conclusion. And then I thought, maybe I have missed something somewhere, so I posted it here.


And, Adne, in response to your suggestion of getSuperClass(), what if A is abstract??
[ November 16, 2005: Message edited by: Stuart Ash ]

[Ådne Brunborg]

Originally posted by Stuart Ash:

And, Adne, in response to your suggestion of getSuperClass(), what if A is abstract??

Then it cannot done.

getClass() returns "the runtime class of an object", and in order to have "A" returned from the getClass()-method, you need an object which is an instance of A without being an instance of any of A's subclasses.

If A is abstract, it cannot be instansiated as A. If you instanciate A as B, getClass() would return the more specific of the two - which is B.

A me = new B();
System.out.print(me.getClass()); //B, not A

[Ernest Friedman-Hill]

Now, recalling that, by definition, "new B() instanceof A" is true, can you tell us why you'd ever want to do this?

[Ådne Brunborg]

I suspect this was more like an academic exercise, "Is this possible or not?" If it had been possible, it can always be used for something

[Hemal Pandya]

Originally posted by Stuart Ash:
Consider class B extends class A.

For any code invoked from B, use of "this" returns an object of type B, such that this.getClass() gives "B".

How can one, if at all possible, get the base class subobject, that is, .getClass() should return A.

I too am curious why you want to do this. For concrete superclass, you might be able to use reflection. The source code of Object{Input,Output}Stream might provide an insight.

[Arun Kumarr]

Hmmm....
Getting more coffee...
[ November 18, 2005: Message edited by: Arun Kumarr ]

[Jim Yingst]

Is this what you're trying to do?

Class getSuperclass(Object obj) {
    return obj.getClass().getSuperclass();
}