trying to communicate between classes

[rippon]

I'm trying to communicate from one class to another, but just can't manage it. I hope the following code is self-explanatory about what I'm trying to do.

Thanks in advance for any replies.

public class SuperPanel extends JPanel {
   public JTextField messageBox = new JTextField("public message box", 20);
   private SubPanel mySP = new SubPanel();

    public SuperPanel() {
      add(messageBox);
      add(mySP);
    } // end of SuperPanel constructor

} // end of: public class SuperPanel extends JPanel


public class SubPanel extends JPanel{

   public SubPanel(){ // I have tried the following options
// to try to communicate with the SuperPanel class
      messageBox.setText("hallo from SubPanel"); //option 1
      super.messageBox.setText("hallo from SubPanel"); //option 2
  } // end of SubPanel() constructor

} // end of SubPanel class

[Junky]

Just like the SuperPanel class has a reference to the subPanel class the reverse has to be true too. How can SubPanel communicate to SuperPanel if it doesn't have any references to a SuperPanel? Here's one way to fix it.

public class SuperPanel extends JPanel {
    private SubPanel mySP = new SubPanel(this); // passes a reference of the SuperPanel to the SubPanel constructor

    public SuperPanel() {
        add(messageBox);
        add(mySP);
    } // end of SuperPanel constructor
} // end of: public class SuperPanel extends JPanel

public class SubPanel extends JPanel{
    SuperPanel superpanel

    public SubPanel(SuperPanel sp) {
        superpanel = sp;
        // call superpanel methods here
    } // end of SubPanel() constructor
} // end of SubPanel class

[rippon]

Thanks.
I believe I have implemented what you say correctly.
It compiles successfully. But I get NullPointerException when I try to call the superpanel method.

[Junky]

My bad, it needs to be inside the constructor.

    private SubPanel mySP;

    public SuperPanel() {
        mySP =  new SubPanel(this); // passes a reference of the SuperPanel to the SubPanel constructor
    }

[j2me64]

But I get NullPointerException when I try to call the superpanel method.

strange. a tested it with this SuperClass

import javax.swing.JPanel;
import javax.swing.JTextField;

public class SuperPanel extends JPanel {
	public JTextField messageBox = new JTextField("public message box", 20);
    private SubPanel mySP = new SubPanel(this); // passes a reference of the SuperPanel to the SubPanel constructor

    public SuperPanel() {
        add(messageBox);
        add(mySP);
    } // end of SuperPanel constructor
    
    public String getMessage() {
    	return messageBox.getText();
    }
    
    public static void main(String[] args) {
		new SuperPanel();
	}
} // end of: public class SuperPanel extends JPanel

and this SubClass

import javax.swing.JPanel;

public class SubPanel extends JPanel {
    SuperPanel superpanel;

    public SubPanel(SuperPanel sp) {
        superpanel = sp;
        // get the message from superpanel
        System.out.println(superpanel.getMessage());
    } // end of SubPanel() constructor
} // end of SubPanel class

and the output was: public message box

this means, the SubPanel could get the message from the SuperClass.

[rippon]

It works! Thanks!
Now, why does it need to be inside the constructor?
I think: Because, before the constructor is executed, no SuperPanel object exists, so no reference to it exists, so 'this' isn't actually pointing to anything - hence the NPE.

Correct?

[Junky]

I thought that the problem was using "this" before the object was actually created but that is not the case. It would seem that OP has done something else incorrectly and since we cannot see the code we have no idea.

[rippon]

What does "OP" stand for? (I presume that's a reference to me.)

[Junky]

Yes, OP = original poster or original post, depending upon context.