如何使用KeyListener在JFrame中移动矩形？

Question

I'm trying to move a rectangle I've created (actually, I'm changing X and Y variables I've created) in a JFrame using a KeyListener. I'd like to state that I do not wish to use keybindings. I would like to use a KeyListener. That being said, I can't seem to find anything wrong with my logic here.

Frame (Main) class:

import java.awt.Color;
import javax.swing.JFrame;

@SuppressWarnings ("serial")
public class GameFrame extends JFrame
{
    public static void main (String[] args)
    {
        JFrame frame = new JFrame ("Java Game");
        frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);
        frame.setSize (1000, 600);
        frame.getContentPane().setBackground (Color.WHITE);
        frame.getContentPane().add (new GamePaint());
        frame.addKeyListener (new GameController());
        frame.setVisible (true);
    }
}

Paint class:

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JComponent;

@SuppressWarnings ("serial")
public class GamePaint extends JComponent
{
    public int x;
    public int y;

    public void paintComponent (Graphics graphics)
    {
        super.paintComponent (graphics);
        graphics.setColor (Color.BLACK);
        graphics.fillRect (x, y, 100, 100);
    }
}

Controller class:

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

public class GameController implements KeyListener
{
    GamePaint paint = new GamePaint();

    public void keyPressed (KeyEvent event)
    {
        if (event.getKeyCode() == KeyEvent.VK_LEFT)
        {
            paint.x--;
            paint.repaint();
        }
        else if (event.getKeyCode() == KeyEvent.VK_RIGHT)
        {
            paint.x++;
            paint.repaint();
        }
    }

    public void keyReleased (KeyEvent event) {}
    public void keyTyped (KeyEvent event) {}
}

Answer 1

You're creating multiple instances of GamePaint ...

frame.getContentPane().add (new GamePaint());

And

public class GameController implements KeyListener
{
    GamePaint paint = new GamePaint();

So when you GameController controller tries to change the x / y positions of the rectangle, it's not interacting with the instance of GamePaint which is actually on the screen...

Instead, create a single instance of GamePaint , add this to the frame and pass a reference to the GameController

GamePaint gp = new GamePaint();
frame.getContentPane().add (gp);
frame.addKeyListener (new GameController(gp));

And update the GameController ...

public class GameController implements KeyListener
{
    private GamePaint paint;
    public GameController(GamePaint paint) {
        this.paint = paint;

But this approach is bunch of weirdness...isn't GamePaint suppose to be receiving keyboard input and not the frame??

Instead, consider using the key bindings API, it solves so many other issues associated with KeyListener . See How to Use Key Bindings for more details

Instead of the controller modifying the x / y properties directly, it should be calling a method on the GamePaint class (something like, moveLeft , moveRight etc...), this way GamePaint can make the decisions about what that means...

But, personally, I'd inject a model in there, which controlled the location of entities and which the GamePaint simply painted and the controller controlled...but that's me...