我不能通过键绑定移动我的JLabel吗？

Question

I am trying to make a game in Java where pressing spacebar moves the box in the window. I am using Key Binding to accomplish this task. The problem is that I can't figure out how to use an ActionListener on the box itself, which is a JLabel. Here is the code below:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;

import javax.swing.AbstractAction; 
import javax.swing.Action;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.KeyStroke;

public class Game {

private static JFrame frame     = new JFrame();
private static JPanel gamePanel = new JPanel();
private static Action playerAction = new PlayerListener();
private static JLabel box = new JLabel();
private static int x = 250;
private static int y = 250;

public static void main(String[] args) {

    frame.add(boxPanel());
    frame.setTitle("Block Game");
    frame.setSize(500,500);
    frame.setLocationRelativeTo(null);
    frame.setFocusable(true);

    box.addActionListener(playerAction);

    frame.setVisible(true);

}

static JPanel boxPanel() {
    ImageIcon boxIcon = new ImageIcon("box.png");
    box.setIcon(boxIcon);

    box.setSize(30,30);
    box.setLocation(x,y);

    box.getInputMap().put(KeyStroke.getKeyStroke("SPACE"), "doPlayerAction");
    box.getActionMap().put("doPlayerAction", playerAction);

    gamePanel.setLayout(null);
    gamePanel.add(box);
    frame.add(gamePanel);

    return gamePanel;   
}

static class PlayerListener extends AbstractAction {

    public void actionPerformed(ActionEvent e) {

        System.out.println("SPACEBAR");

    }

}
}

I tried changing the box to a JButton and working with that instead, but I have found that "SPACEBAR" only prints out when I click on the box itself. Any help is greatly appreciated. Thanks!

Answer 1

Your "core" problem revolves around box.getInputMap() , change it to something more like box.getInputMap(WHEN_IN_FOCUSED_WINDOW) which will mean that the API will respond to key events whenever the window has focus, regardless what other components might have focus.

I'd also suggestsomething more like box.getInputMap(WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_SPACE, 0), "doPlayerAction") , as the mechanism which KeyStroke uses to parse String to a KeyStroke is more complicated then it might seem, normally requiring additional information like pressed , released or typed , it's just easier to use the virtual keys

I would also bind the keys to the gamePanel as a general preference, as it should be the container making decisions about what to do, but that's just me.

Have a look at How to Use Key Bindings for more details