Java使用单独的类移动图像

Question

我正在制作一个小型的爱好游戏，到目前为止我有几节课。有两个课程我遇到了一些问题，下面列出了有问题的课程。

屏幕.Java

package geisst.flat;

import javax.imageio.ImageIO;
import javax.swing.JFrame;

import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

public class Screen extends JFrame {

public int x = 100;
public int y = 100;

public Screen() {
    this.setSize(400, 400);
    this.setTitle("Flat Game");
    this.setResizable(false);
    this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    this.setVisible(true);

    addKeyListener(new KeyListen());
}

public void paint(Graphics g) {
    BufferedImage mainImage = null;

    try {
        mainImage = ImageIO.read(new File("res/test.gif"));
    } catch(IOException e) {

    }

    g.drawImage(mainImage, x, y, null);
    repaint();
}
}

还有 KeyListen.Java

package geisst.flat;

import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

public class KeyListen extends KeyAdapter {

Screen screen;

@Override
public void keyPressed(KeyEvent e) {
    int keyCode = e.getKeyCode();

    if(keyCode == KeyEvent.VK_LEFT) {
        screen.x += 3;
    }
}

}

KeyListen 应该将 mainImage 的 x 位置向上移动 3 个像素，但我收到以下错误。

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at geisst.flat.KeyListen.keyPressed(KeyListen.java:15)
at java.awt.Component.processKeyEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Window.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.KeyboardFocusManager.redispatchEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

你们知道为什么会这样吗？这可能是非常明显的事情：P

提前致谢，

盖斯特

Answer 1

您永远不会初始化screen 的值。

你可以这样做：

在KeyListener 类中

public KeyListen (Screen screen) {
 this.screen = screen;
}

在Screen 类中：

addKeyListener(new KeyListen(this));

Answer 2

你从不初始化屏幕，所以它是空的，但你使用它的属性：

screen.x += 3;

您应该将Screen 对象的引用传递给侦听器，或者通过调用getSource：

@Override
public void keyPressed(KeyEvent e) {
    int keyCode = e.getKeyCode();

    if(keyCode == KeyEvent.VK_LEFT) {
        if (e.getSource() instanceof Screen)
            ((Screen)e.getSource()).x += 3;
    }
}

Answer 3

1) 不好直接画到JFrame上，放JPanel、JComponent或者JLabel

2) 对于 Swing JComponents，是否有paintComponent 而不是paint，paint 是绘制到RootPane 或GlassPane（从JFrame 派生的组件）的正确方法

3) 使用 KeyBindings 而不是 KeyListener，因为 KeyListener 在窗口中没有焦点时不起作用，将焦点设置为 ContentPane 不是一个好主意

4) 将 JComponent 用于 Custom Painting 或 JLabel 和 Icon

5) KeyBindings 示例here