JApplet & KeyListener | 易学教程

问题

I'm developing an application for my computer science classes. The task is to write a calculator but without using JTextFields or JTextAreas. I've come up with an idea of implementing KeyListener which works nice in both appletviewer and JFrame but doesn't work at all in Google Chrome (and probably other browsers).

Here're my code snippets.

//- BinaryCalc.java
import javax.swing.*;
import javax.swing.event.*;
import java.awt.*;
import java.awt.event.*;

public class BinaryCalc extends JApplet implements KeyListener {

    private JPanel panel;

    public BinaryCalc() {
        super();

        panel = new JPanel();
        this.add(panel);

        panel.addKeyListener(this);
        panel.requestFocusInWindow();
    }

    @Override
    public void init() {
        JOptionPane.showMessageDialog(this, "applet");
        panel.setFocusable(true);
        panel.requestFocus();
    }

    public void keyPressed(KeyEvent e) {
        JOptionPane.showMessageDialog(this, (char) e.getKeyCode());
    }

    public void keyReleased(KeyEvent e) {}

    public void keyTyped(KeyEvent e) {}

    public JPanel getPanel() { return panel; }

    public static void main(String args[]) {
        JFrame frame = new JFrame("Binary Calculator");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setMinimumSize(new Dimension(320, 240));

        BinaryCalc kalkulator = new BinaryCalc();
        frame.add(kalkulator);

        frame.pack();
        frame.setVisible(true);
        kalkulator.getPanel().requestFocusInWindow();
    }

}

And HTML file containing my applet.

<!doctype html>
<html>

<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <link rel="stylesheet" type="text/css" href="style.css">
    <title>Kalkulator binarny</title>
</head>

<body>
    <div style="text-align: center; border-bottom: 1px solid black">
        <h1>Kalkulator Binarny</h1>
    </div>

    <br/>
    <object style="display: block; margin: auto;" type="application/x-java-applet" width="320" height="240">
    <param name="code" value="BinaryCalc.class" />
    <!--- <param name="archive" value="Liczba.jar" /> -->
        What a terrible failure: applet failed to load!
    </object>   
    <br/>
</body>

</html>

Any ideas?

回答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");
            }
        });
    }
}

回答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");
            }
        });
    }
}

来源：https://stackoverflow.com/questions/13363208/japplet-keylistener

标签

java

swing

jframe

keylistener

japplet