使用箭头键移动矩形

Question

I am trying to develop a simple game where you can move a rectangle using the left and right arrow keys and to shoot using the spacebar.

I have added a KeyListener and others, but when I run it I have nothing as the output of key pressed or others.

I have two classes:

PaintDemo class:

package game;

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

public class PaintDemo {

    public static void main(String[] args) {

        JFrame mainFrame = new JFrame("Game 1.1");
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        mainFrame.setBounds(10, 10, 400, 400);
        mainFrame.setLayout(new BorderLayout());

        JPanel mainPanel = new JPanel();
        mainPanel.setBackground(Color.WHITE);

        PaintComponent paintPanel = new PaintComponent();

        mainFrame.add(mainPanel, BorderLayout.PAGE_START);
        mainFrame.add(paintPanel, BorderLayout.CENTER);

        mainFrame.setVisible(true);

    }
}

PaintComponent class:

package game;

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.geom.Rectangle2D;
import javax.swing.JPanel;


public class PaintComponent extends JPanel implements KeyListener {
    int dx = 200;
    int dy = 300;
    int my = 300;
    public Rectangle2D rec = new Rectangle2D.Double(dx , dy, 30, 10);

    public PaintComponent() {
        this.addKeyListener(this);
        this.setBackground(Color.white);
    }

    public void shoot(KeyEvent evt){
        if (evt.getKeyCode() == KeyEvent.VK_SPACE){
        my -= 7;
        repaint();
        }
    }

    public void moveRec(KeyEvent evt){
        switch(evt.getKeyCode()){
            case KeyEvent.VK_LEFT:
                System.out.println("test");
                dx -= 2;
                rec.setRect(dx, dy, 30, 10);
                repaint();
            case KeyEvent.VK_RIGHT:
                dx += 2;
                rec.setRect(dx, dy, 30, 10);
                repaint();
        }
    }

    @Override
    protected void paintComponent(Graphics grphcs){
        super.paintComponent(grphcs);
        Graphics2D gr = (Graphics2D) grphcs;
        gr.draw(rec);
    }


    @Override
    public void keyTyped(KeyEvent e) {
        System.out.println("2");
        shoot(e);
    }


    @Override
    public void keyPressed(KeyEvent e) {
        moveRec(e);

    }

    @Override
    public void keyReleased(KeyEvent e) {

    }
}

The shooting method is not complete.

Answer 1

Change repaint() to paintComponent here and add Graphics g = getGraphics();

public void moveRec(KeyEvent evt){
    switch(evt.getKeyCode()){
        case KeyEvent.VK_LEFT:
            System.out.println("test");
            dx -= 2;
            rec.setRect(dx, dy, 30, 10);
            Graphics g = getGraphics();
            paintComponent(g);
            break;
        case KeyEvent.VK_RIGHT:
            dx += 2;
            rec.setRect(dx, dy, 30, 10);
            Graphics g = getGraphics();
            paintComponent(g);
            break;
    }
}    

Let me know if this works :)

EDIT:
Add this line

this.setFocusable(true);

to PaintComponent constructor, it will make keypresses go through.

Answer 2

You forgot to do two things, First one is your panel should be focusable, add this in your PaintComponent constructor this.setFocusable(true) and the second one is you have not break the switch case, so add the break statement for each case, then it should work.

switch (evt.getKeyCode()) {
case KeyEvent.VK_LEFT:
    System.out.println("test");
    dx -= 2;
    rec.setRect(dx, dy, 30, 10);
    repaint();
    break;
case KeyEvent.VK_RIGHT:
    dx += 2;
    rec.setRect(dx, dy, 30, 10);
    repaint();
    break;
}