How to get change a GUI component from another class?

Question

Hi there,
I'm currently trying to change a GUI component in my 'Application' class from my 'Dice' class.
So the Application class sets up some GUI including a JLabel that initially displays "Change".
The 'Dice' class contains the ActionPerformed() method for when the 'Change' button (made from Application class) is clicked.
And it returns an 'int' between 1 and 6.
Now I want to set this number back int he JLabel from the Application class.
APPLICATION CLASS
import javax.swing.*;
import java.awt.*;
import java.util.Random;
import java.awt.event.*;
public class Application extends JFrame implements ActionListener{
     public JPanel rollDicePanel = new JPanel();
     public JLabel dice = new JLabel("Loser");
     public Container contentPane = getContentPane();
     public JButton button = new JButton("Change");
     public Dice diceClass = new Dice();
     public Application() {}
     public static void main(String[] args)
          Application application = new Application();
          application.addGUIComponents();
     public void addGUIComponents()
          contentPane.setLayout(new BorderLayout());
          rollDicePanel.add(dice);
        button.addActionListener(diceClass);
        contentPane.add(rollDicePanel, BorderLayout.SOUTH);
        contentPane.add(button,BorderLayout.NORTH);
          this.setSize(460, 655);
          this.setVisible(true);
          this.setResizable(false);
     public void changeDice()
          dice.setText("Hello");
     public void actionPerformed(ActionEvent e) {}
}DICE
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Dice implements ActionListener
     public Dice() {}
     public void actionPerformed(ActionEvent e)
          //super.actionPerformed(e);
          String event = e.getActionCommand();
          if(event.equals("Change"))
               System.out.println("Will be about to change the 'dice' label");
               Application application = new Application();
               application.dice.setText("Hello");
}

Answer 1

It's all about references, baby. The Dice object needs a way to communicate with the Application object, and so Dice needs a reference to Application. There are many ways to pass this. In my example I pass the application object directly to Dice, but a better way would use interfaces and some indirection. Look up the Observer pattern for a better way to do this that scales much better than my brute-force approach.
import javax.swing.*;
import java.awt.*;
public class Application extends JFrame // *** implements ActionListener
    // *** make all of these fields private ***
    private JPanel rollDicePanel = new JPanel();
    private JLabel dice = new JLabel("Loser");
    private Container contentPane = getContentPane();
    private JButton button = new JButton("Change");
    // *** pass a reference to your application ("this")
    // *** to your Dice object:
    private Dice diceClass = new Dice(this);
    public Application()
    public static void main(String[] args)
        Application application = new Application();
        application.addGUIComponents();
    public void addGUIComponents()
        contentPane.setLayout(new BorderLayout());
        rollDicePanel.add(dice);
        button.addActionListener(diceClass);
        contentPane.add(rollDicePanel, BorderLayout.SOUTH);
        contentPane.add(button, BorderLayout.NORTH);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setPreferredSize(new Dimension(460, 655));
        pack();
        setLocationRelativeTo(null);
        setVisible(true);
        setResizable(false);
    // *** I'm not sure what this is supposed to be doing, so I commented it out.
    //public void changeDice()
        //dice.setText("Hello");
    // *** ditto.  I strongly dislike making a GUI class implement ActionListeenr
    //public void actionPerformed(ActionEvent e)
    // *** here's the public method that the Dice object calls
    public void setTextDiceLabel(String text)
        dice.setText(text);
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
public class Dice implements ActionListener
    // *** have a variable that holds a reference to your application object
    private Application application;
    private boolean hello = true;
    public Dice(Application application)
        // *** get that reference via a constructor parameter (one way to do this)
        this.application = application;
    public void actionPerformed(ActionEvent e)
        String event = e.getActionCommand();
        if (event.equals("Change"))
            System.out.println("Will be about to change the 'dice' label");
            if (hello)
                // *** call the application's public method
                application.setTextDiceLabel("Hello");
            else
                application.setTextDiceLabel("Goodbye");
            hello = !hello;
            //Application application = new Application();
            //application.dice.setText("Hello");
}