在jframe

Question

我想尝试在jframe中按键上按下矩形。 但是当按下向上或向下箭头键并且它没有停止时，矩形只会下降。我无法看到我犯了什么错误。

我不认为这个错误存在于文件一中，但正如所说，我无法找到它，所以它可能就是。

档案1

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

public class test{

    public static void main (String[] arg) {
    JFrame window = new JFrame();
        test2 t2 = new test2();
        window.add(t2);
        window.setSize(1000,1000);
        window.setTitle("TEST");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        window.setVisible(true);
    }
}

文件2

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

public class test2 extends JPanel implements ActionListener, KeyListener{

    Timer t = new Timer(5, this);
    double x = 0, y = 0, velx = 0, vely = 0;

    public test2() {
        t.start();
        addKeyListener(this);
        setFocusable(true);
        setFocusTraversalKeysEnabled(false);
    }



    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.fill(new Rectangle((int)x, (int)y, 20, 40));
    }

    public void actionPerformed(ActionEvent e) {
        repaint();
        x += velx;
        y += vely;
    }

    public void up() {
        vely = -1.5;
        velx = 0;
    }

    public void down() {
        vely = 1.5;
        velx = 0;   
    }

    public void keyPressed(KeyEvent e) {
        int code = e.getKeyCode();

        if (code == KeyEvent.VK_UP); {
            up();
        }

        if (code == KeyEvent.VK_DOWN); {
            down();
        }
    }

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

Answer 1

你的逻辑不是很理想。您的代码应该更像这样：

/**  DELETE THIS METHOD!
public void actionPerformed(ActionEvent e) {
    repaint();
    x += velx;
    y += vely;
}**/

private static final double MOVEMENT = 1.5;

public void up() {
    x += -MOVEMENT; // Mind the - in front of MOVEMENT to negate the value.
    // No need to change y if it does not change at all

    repaint() // Repaint _after_ you changed the coordinates
}

public void down() {
    x += MOVEMENT; // Mind that there is no - infront of this MOVEMENT.
    // No need to change y if it does not change at all

    repaint() // Repaint _after_ you changed the coordinates
}

public void keyPressed(KeyEvent e) {
    int code = e.getKeyCode();

    if (code == KeyEvent.VK_UP); {
        up();
    }

    if (code == KeyEvent.VK_DOWN); {
        down();
    }
}

Answer 2

在改变x，y;

的位置后，你错过了在你的JPanel上调用repaint（）

另请注意，您的事件只会触发两次（OnKeyDown和OnKeyUp）。