JApplet & KeyListener

Question

I\'m developing an application for my computer science classes. The task is to write a calculator but without using JTextFields or JTextArea<

Answer 1

• Use KeyBindings and not KeyListener

• Do not create your own JFrame for a JApplet simply call getContentPane() on applet instance and add all your components there.

• All components should be created in JApplet overriden init() method wrapped in a SwingUtilities#invokeAndWait(..) block

• JApplets and Applets do not have a main(..) method (besides testing purposes)

• Use requestFocusInWindow() instead of requestFocus()

I would highly suggest you read:

• How to Make Applets
• JApplet tutorial

Here is an example works for me. It simply adds an un-editable JTextField to the JPanel and then adds KeyBindings for KeyEvent.VK_0 and KeyEvent.VK_1 to the JPanel. If the user types either 0 or 1 it will be displayed in un-edtiable JTextField:

import java.awt.BorderLayout;
import java.awt.event.*;
import java.lang.reflect.InvocationTargetException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.*;

public class BinaryCalc extends JApplet {

    private JTextField jtf;

    @Override
    public void init() {
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                @Override
                public void run() {

                    JPanel panel = new JPanel();
                    setKeyBindings(panel);
                    jtf = new JTextField(10);
                    //so we cant edited it without pressing a key
                    jtf.setEditable(false);

                    panel.add(jtf);
                    getContentPane().add(panel, BorderLayout.CENTER);
                    panel.requestFocusInWindow();//incase we lost focus
                }
            });
        } catch (InterruptedException ex) {
            Logger.getLogger(BinaryCalc.class.getName()).log(Level.SEVERE, null, ex);
        } catch (InvocationTargetException ex) {
            Logger.getLogger(BinaryCalc.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    private void setKeyBindings(final JPanel panel) {
        panel.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_0,0), "0");
        panel.getActionMap().put("0", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                String tmp = jtf.getText();
                jtf.setText(tmp + "0");
            }
        });
        panel.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_1,0), "1");
        panel.getActionMap().put("1", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                String tmp = jtf.getText();
                jtf.setText(tmp + "1");
            }
        });
    }
}

Answer 2

I took the liberty of converting @David's helpful example to use a label and the numeric keypad.

Update: This hybrid works on Ubuntu/OpenJDK, and it can be deployed via Java Web Start.

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.EventQueue;
import java.awt.event.*;
import javax.swing.*;

/**
 * @see https://stackoverflow.com/a/13363349/230513
 */
public class BinaryCalc extends JApplet {

    private static BinaryCalc bc = new BinaryCalc();
    private JLabel label = new JLabel("0000000000");

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

            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.setTitle("BinaryCalc");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                initContainer(frame);
                frame.pack();
                bc.label.setText("");
                frame.setVisible(true);
            }
        });
    }

    // Common initialization for either JApplet or JFrame
    private static void initContainer(Container container) {
        JPanel panel = new JPanel();
        bc.setKeyBindings(panel);
        panel.add(bc.label);
        container.add(panel, BorderLayout.CENTER);
        panel.requestFocusInWindow();
    }

    @Override
    public void init() {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                initContainer(BinaryCalc.this);
            }
        });
    }

    private void setKeyBindings(final JPanel panel) {
        panel.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW)
            .put(KeyStroke.getKeyStroke(KeyEvent.VK_0, 0), "0");
        panel.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW)
            .put(KeyStroke.getKeyStroke(KeyEvent.VK_NUMPAD0, 0), "0");
        panel.getActionMap().put("0", new AbstractAction() {

            @Override
            public void actionPerformed(ActionEvent ae) {
                String tmp = label.getText();
                label.setText(tmp + "0");
            }
        });
        panel.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW)
            .put(KeyStroke.getKeyStroke(KeyEvent.VK_1, 0), "1");
        panel.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW)
            .put(KeyStroke.getKeyStroke(KeyEvent.VK_NUMPAD1, 0), "1");
        panel.getActionMap().put("1", new AbstractAction() {

            @Override
            public void actionPerformed(ActionEvent ae) {
                String tmp = label.getText();
                label.setText(tmp + "1");
            }
        });
    }
}