KeyListener

[Matthew Middleton]

I have an application that is set up like this, the main frame is a jframe, I have a menu bar on it and two jpanels on the inside of the jframe, one to hold the textfields and one to hold my buttons, previous and next. I want to add keylisteners to the application so that when you press the right arrow key it would be the same as pressing the next button and the left arrow key the same as the Previous button. What part of my app (the jframe, the buttons or the jpanels) should I add the keylisteners too?? I think I've tried them all, but it seems like it does nothing do I have to do something else since I have so many panels, add a focus to one of them, or maybe the buttons?? Any help here would be appriciated. Thank you for your time and GOD Bless.

Matthew Middleton

[Gregg Bolinger]

Well, here is the problem. There is no such thing as a Global KeyListener in Java. The only way to handle this is to add a KeyListener to every single component in your JFrame so that every component is listening for KeyEvents.

This is a lot easier than it sounds, luckily. Take a look at this JavaWorld article for a really good and simple example.

I did some testing with it and it worked great. Something else I didn't know before reading that article and testing that is helpful is that the Arrow Keys don't register keyTyped events. Only keyPressed and keyReleased so make sure your code that handles those events are in the right KeyListener methods.

[David Weitzman]

In whatever code initializes the JButtons (Next, previous, etc.) you should be able to do something like this:

JButton next = ...
 
next.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("LEFT"), "left");
next.getActionMap().put("left", new AbstractAction(){
  public void actionPerformeced(ActionEvent evt) {
    next.doClick();
  }
});

I've found that KeyListener's are appropriate for situations when you want to be able to handle arbitrary input, like if you're writing a text component that accepts characters of any kind. Much more commonly I use InputMaps, which are there to handle binding a specific key to an action and can easily watch all keystrokes sent to the window or to sub-components of a parent component. Check out How to Use Key Bindings in the Java Tutorial for more info.

[Michael Dunn]

If you want something to play around with, try this.

Problem is it fires 2 or 3 times - I'm guessing once for each of key-Pressed/Typed/Released
(I don't have a lot of time at present to work it out)

If you can filter out the multiple firing, it could be what you're after.

Good luck

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
class Testing extends JFrame implements ActionListener
{
  JButton btnFocus = new JButton("Take focus");
  JButton btnPrevious = new JButton("Previous");
  JButton btnNext = new JButton("Next");
  JTextField tf1 = new JTextField(5);
  JTextField tf2 = new JTextField(5);
  public Testing()
  {
    setTitle("Testing");
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setLocation(400,300);
    setSize(400,75);
    Container frame = getContentPane();
    KeyboardFocusManager.getCurrentKeyboardFocusManager()
     .addKeyEventDispatcher ( new KeyEventDispatcher()  {
       public boolean dispatchKeyEvent(KeyEvent e){
          int key = e.getKeyCode();
          if(key == 37 || key == 100) previous();
          else if(key == 39 || key == 102) next();
          return false;}
    });
    JPanel jp = new JPanel();
    btnPrevious.addActionListener(this);
    btnNext.addActionListener(this);
    jp.add(btnFocus);
    jp.add(btnPrevious);
    jp.add(btnNext);
    jp.add(tf1);
    jp.add(tf2);
    frame.add(jp);
  }
  public void actionPerformed(ActionEvent ae)
  {
    if(ae.getSource() == btnPrevious) previous();
    else next();
  }
  public void previous(){JOptionPane.showMessageDialog(this,"code for previous");}
  public void next(){JOptionPane.showMessageDialog(this,"code for next");}
 
  public static void main( String[] args){new Testing().setVisible(true);}
}

[Craig Wood]

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class KeyActionsTest
{
    public static void main(String[] args)
    {
        KeyActionsPanel panel = new KeyActionsPanel();
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setJMenuBar(panel.getMenuBar());
        f.getContentPane().add(panel);
        f.getContentPane().add(panel.getSouthPanel(), "South");
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
    }
}
 
class KeyActionsPanel extends JPanel
{
    JTextField[] fields;
    int fieldIndex;
 
    public KeyActionsPanel()
    {
        fields = new JTextField[4];
        fieldIndex = 0;
        setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.weighty = 1.0;
        gbc.gridwidth = gbc.REMAINDER;
        for(int j = 0; j < fields.length; j++)
        {
            fields[j] = new JTextField(12);
            add(fields[j], gbc);
            fields[j].getInputMap().put(KeyStroke.getKeyStroke("LEFT"), "BACK");
            fields[j].getActionMap().put("BACK", back);
            fields[j].getInputMap().put(KeyStroke.getKeyStroke("RIGHT"), "NEXT");
            fields[j].getActionMap().put("NEXT", ahead);
        }
        setBackground(Color.white);
    }
 
    public JMenuBar getMenuBar()
    {
        JMenu menu = new JMenu("menu");
        menu.add(back);
        menu.add(ahead);
        JMenuBar menuBar = new JMenuBar();
        menuBar.add(menu);
        return menuBar;
    }
 
    public JPanel getSouthPanel()
    {
        JButton
            last = new JButton("previous"),
            next = new JButton("next");
         last.setAction(back);
         next.setAction(ahead);
         JPanel panel = new JPanel();
         panel.add(last);
         panel.add(next);
         return panel;
    }
 
    private Action back = new AbstractAction("previous")
    {
        public void actionPerformed(ActionEvent e)
        {
            fieldIndex--;
            if(fieldIndex < 0)
                fieldIndex = fields.length - 1;
            fields[fieldIndex % fields.length].requestFocusInWindow();
        }
    };
 
    private Action ahead = new AbstractAction("next")
    {
        public void actionPerformed(ActionEvent e)
        {
            fieldIndex++;
            fields[fieldIndex % fields.length].requestFocusInWindow();
        }
    };
}

[Gregg Bolinger]

So can we all agree that one downfall of Swing is that there are too many ways to do the same thing?

[P. Sagdeo]

That's programming, or even math. 1 + 2 = 3, 2 + 1 =3, 3 = 2 + 1, et.

Anyway, why can't you have the listener in any component, and just requestFocus()?