Clicking a JButton from keyboard using KeyListener

Question

Like I described in the title, I am required to make a JButton clicked if a key from a keyboard is pressed. For example :

ActionListenerClass actionListener = new ActionListenerClass();
KeyListenerClass actionListener = new KeyListenerClass();
JButton aButton = new JButton("A");
aButton.setActionCommand("A");
aButton.addActionListener(actionListener);
aButton.addKeyListener(keyListener);

When "A" is pressed from the keyboard, button A will perform a doClick() and send the action command to a private class of the action listener for event handling. Now I have read a lot of solution from stack overflow, and they all used key binding, which is to bind between an input map and an action map. The thing is I absolutely have to use a key listener with a private class but not the binding. The only thing I can guess now is the keyListener above have to somehow receive the keyboard input and perform doClick on the button it is binded to in a keyPressed method, which I have try and it didn't work at all.

Edit: Here's my entire code.

1. CalculatorViewController.java

import java.awt.*;
import java.awt.event.*;
import java.util.regex.Pattern;

import javax.swing.*;
/**Create the app GUI
 * @author Bach Le
 * @version 1.0
 * @see java.awt, java.awt.event, javax.swing
 * @since 12.0.1
 */
public class CalculatorViewController extends JPanel   {

    
    private JButton backSpaceButton;
     public CalculatorViewController() {
    Controller controller = new Controller();
    KeyController keyController  = new KeyController();

    
    
    setBorder(BorderFactory.createMatteBorder(5, 5, 5, 5,Color.black));//Adding the panel border
    
    
     backSpaceButton = new JButton("\u21DA");

    backSpaceButton.setPreferredSize(new Dimension(52,55));
    backSpaceButton.setOpaque(false);//set transparency
    backSpaceButton.setContentAreaFilled(false);
    backSpaceButton.setBorderPainted(false);
    backSpaceButton.setActionCommand("Backspace Button");//set the action command
    backSpaceButton.addActionListener(controller);//add action listener
    backSpaceButton.setToolTipText("Backspace (Alt+B)");//set tooltip text
    backSpaceButton.setFont(font);//set the font
    backSpaceButton.addKeyListener(keyController);
   
   add(backSpaceButton) ;
   
 
   



}
    


private class Controller implements ActionListener{
public void actionPerformed(ActionEvent e) {
//event handling here
}
}

private class KeyController implements KeyListener{

    @Override
    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub
    
    }

    @Override
    public void keyPressed(KeyEvent e) {
    if(e.getKeyCode()==65) {
        backSpaceButton.doClick();
        
    }
        
    }

    @Override
    public void keyReleased(KeyEvent e) {
        // TODO Auto-generated method stub
        
    }
    
    
    
    
    
}

}

2.Calculator.java

public class Calculator {
    public static void main(String[] args) {
          CalculatorViewController pane = new CalculatorViewController();
            
             JFrame frame = new JFrame("Calculator");
           frame.setContentPane(pane);  
            frame.setSize(380, 520);
            frame.setLocationByPlatform(true);
            frame.setResizable(true);
            frame.setVisible(true);
       }

}

Focus on CalculatorViewController, I am trying to make backSpaceButton clicked when A is pressed (Of course it is the actual backspace button but I will fix it later), so it will send its action command to the action listener registered to it, which will be processed in the method of Controller inner class. I am not sure the proper way to achieve this.

Answer 1

Adding a KeyListener will only work for components with focus. You would not want to add the KeyListener to the JButtons, because only one JButton will be in focus.

For just setting a value you could use setMnemonic but then you would have to use a modifier (such as 'alt) when you press the key.

The 'correct' way to do this is to use key bindings

Here is an example with two buttons.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class ButtonKeys{
    
    public void buildGui(){
        JFrame frame = new JFrame("key buttons");
        JPanel panel = new JPanel(new BorderLayout());
        JButton a = new JButton("A");
        a.addActionListener(evt->{ System.out.println("a pressed");});
        JButton b = new JButton("B");
        b.addActionListener(evt->{ System.out.println("b pressed");});
        
        panel.add(a, BorderLayout.EAST);
        panel.add(b, BorderLayout.WEST);
        
        frame.setContentPane(panel);
        frame.setVisible(true);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        KeyStroke us = KeyStroke.getKeyStroke(KeyEvent.VK_A, 0, false);
        panel.getInputMap().put(us, "A");
        panel.getActionMap().put("A", new AbstractAction(){
            @Override
            public void actionPerformed(ActionEvent evt){
               a.doClick();
            }
        });
        
        KeyStroke us2 = KeyStroke.getKeyStroke(KeyEvent.VK_B, 0, false);
        panel.getInputMap().put(us2, "B");
        panel.getActionMap().put("B", new AbstractAction(){
            @Override
            public void actionPerformed(ActionEvent evt){
               b.doClick();
            }
        });
        a.setFocusable(false);
        b.setFocusable(false);

    }

    public static void main(String[] args){
        EventQueue.invokeLater( new ButtonKeys()::buildGui);
    }
}

I made the buttons to not obtain focus because if they get focus then their input map will be used.

Answer 2

This code should work for you. For the KeyListener I used a KeyAdapter more here as it is more convenient if you simply need to use one of the methods. You can of course move the Listeners to separate own classes if needed, but the behavior would stay the same.

public static void main(String[] args) {

    JFrame frame = new JFrame();
    JPanel panel = new JPanel();
    
    JButton btn = new JButton("A");
    btn.addActionListener(new ActionListener() {
        
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("Button clicked");
        }
    });
    btn.addKeyListener(new KeyAdapter() {
        public void keyTyped(KeyEvent e) {
            if (e.getKeyChar() == 'A' || e.getKeyChar() == 'a') {
                ((JButton) e.getSource()).doClick();
            }
        }
    });
    panel.add(btn);
    
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setBounds(0, 0, 800, 600);
    frame.getContentPane().add(panel);
    frame.setVisible(true);
}

If you really need to use a KeyListener, it would look like this:

    btn.addKeyListener(new KeyListener() {
        
        @Override
        public void keyTyped(KeyEvent e) {
            if (e.getKeyChar() == 'A' || e.getKeyChar() == 'a') {
                ((JButton) e.getSource()).doClick();
            }
        }
        
        @Override
        public void keyReleased(KeyEvent e) {
            // TODO Auto-generated method stub
            
        }
        
        @Override
        public void keyPressed(KeyEvent e) {
            // TODO Auto-generated method stub
            
        }
    });