Question

I have a class of buttons called Keys.java which returns a panel of buttons to the class called Control.java. I have a JLabel in Control.java, but what I want to do is change a JLabel when a button is pressed. How would you go about doing this?

I have tried setting a string in Keys.java which changes based on the button and then setting the JLabel’s text equal to the string but it doesn’t seem to work.

Any thoughts on how to achieve this?

Answer 1

It may be that you are updating the wrong string or setting the corresponding label’s text incorrectly. Both are required. In the example below (using your names), the two updates are tightly coupled in the button’s actionPerformed(). A more loosely coupled approach is shown here.

import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import javax.swing.AbstractAction;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

/** @see https://stackoverflow.com/questions/9053824 */
public class JavaGUI extends JPanel {

    private Control control = new Control();
    private Keys keys = new Keys("Original starting value.");

    public JavaGUI() {
        this.setLayout(new GridLayout(0, 1));
        this.add(keys);
        this.add(control);
    }

    private class Control extends JPanel {

        public Control() {
            this.add(new JButton(new AbstractAction("Update") {

                @Override
                public void actionPerformed(ActionEvent e) {
                    System.out.println("Command: " + e.getActionCommand());
                    keys.string = String.valueOf(System.nanoTime());
                    keys.label.setText(keys.string);
                }
            }));
        }
    }

    private class Keys extends JPanel {

        private String string;
        private JLabel label = new JLabel();

        public Keys(String s) {
            this.string = s;
            label.setText(s);
            this.add(label);
        }
    }

    private void display() {
        JFrame f = new JFrame("JavaGUI");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(this);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new JavaGUI().display();
            }
        });
    }
}