How to identify "Alt +" keypress, properly and reliably?

Question

My question is actually more general, but I'm using the action of a user holding down the "Alt" key, and pressing "+", as an example that shows the difficulties.

I'm working an a US English keyboard, which has the "=" (lowercase) and "+" (uppercase) on the same key, so to press "Alt +" (as might be indicated in a menu entry), I have to actually press "Alt Shift =". In Java AWT, pressing "Alt Shift =" generates a key-pressed KeyEvent with the keycode associated with the "=" key, and a key-typed KeyEvent containing the "±" character. So there is no obvious, reliable way of programatically deciding that "Alt" was held down while the "+" key was pressed.

I could do some mapping internally to fix this, such as mapping "±" to "Alt +", or mapping "Shift {keycode for = }" to "+". However, there don't seem to be any guarantees that this would work across different keyboard layouts; and it certainly isn't good coding style.

If anyone can suggest a way around these problems, or perhaps point be to code that's already handled this difficulty, I'd be most appreciative.

Thanks.

Answer 1

Try this:

if(e.isAltDown())
{
    switch(e.getKeyChar())
    {
        case '+':
            System.out.println("Plus");
        break;
    }
}

Where e is the KeyEvent and it is handled in keyPressed method.

The above code will print Plus when you press ALT+Shift+= on the keyboard specified by you.

For complete working code see the below example:

import java.awt.Toolkit;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.UIManager;


public class SwingTest 
{

    private static JFrame frame;

    public static void main(String[] args) throws Exception
    {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (Exception e) {
            try {
                UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
            } catch (Exception ex) {
                ex.printStackTrace();
            }
        }

        frame = new JFrame("Event Test");

        Toolkit tk = Toolkit.getDefaultToolkit();  
        int xSize = ((int) tk.getScreenSize().getWidth()/2) + 100;  
        int ySize = ((int) tk.getScreenSize().getHeight()/2) + 50;  

        frame.setSize(xSize,ySize); 
        frame.setLocationRelativeTo(null); 
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.addKeyListener(new KeyListener() 
        {
            public void keyTyped(KeyEvent e) {
            }

            public void keyReleased(KeyEvent e) {
            }

            public void keyPressed(KeyEvent e) 
            {
                if(e.isAltDown())
                {
                    switch(e.getKeyChar())
                    {
                    case '+':
                        System.out.println("Plus");
                        break;
                    }
                }
            }
        });

        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                frame.setVisible(true);
            }
        });         
    }
}

Hope this will help.