==原代码==
import java.awt.*;
import java.awt.event.*;public class TankClient extends Frame {
int x, y = 50;
public static final int GAME_WIDTH = 800;
public static final int GAME_HEIGHT = 600;
//A Image offScreenImage = null;
public void paint(Graphics g) {
Color c = g.getColor();
g.setColor(Color.red);
g.fillOval(x, y, 30, 30);
g.setColor(c);
y += 5;
}
public void update(Graphics g) {
/* B if(offScreenImage == null) {
offScreenImage = this.createImage(GAME_WIDTH, GAME_HEIGHT);
} */
Graphics imageG = offScreenImage.getGraphics();
Color c = imageG.getColor();
imageG.setColor(Color.green);
imageG.fillRect(0, 0, GAME_WIDTH, GAME_HEIGHT);
imageG.setColor(c);
paint(imageG);
g.drawImage(offScreenImage, 0, 0, null);
} public void launchFrame() {
setBounds(400, 300, GAME_WIDTH, GAME_HEIGHT);
setVisible(true);
setResizable(false);
setTitle("TankWar");
setBackground(Color.green);
addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
new Thread(new PaintThread()).start();
}
private class PaintThread implements Runnable {
public void run() {
while(true) {
repaint();
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void main(String[] args) {
TankClient tc = new TankClient();
tc.launchFrame();
}}将原代码中的A句和B段去掉,在A处直接写Image offScreenImage = this.createImage(GAME_WIDTH, GAME_HEIGHT);
为何运行后小球就不向下走了,分析下是哪里的问题吧?
import java.awt.*;
import java.awt.event.*;public class TankClient extends Frame {
int x, y = 50;
public static final int GAME_WIDTH = 800;
public static final int GAME_HEIGHT = 600;
//A Image offScreenImage = null;
public void paint(Graphics g) {
Color c = g.getColor();
g.setColor(Color.red);
g.fillOval(x, y, 30, 30);
g.setColor(c);
y += 5;
}
public void update(Graphics g) {
/* B if(offScreenImage == null) {
offScreenImage = this.createImage(GAME_WIDTH, GAME_HEIGHT);
} */
Graphics imageG = offScreenImage.getGraphics();
Color c = imageG.getColor();
imageG.setColor(Color.green);
imageG.fillRect(0, 0, GAME_WIDTH, GAME_HEIGHT);
imageG.setColor(c);
paint(imageG);
g.drawImage(offScreenImage, 0, 0, null);
} public void launchFrame() {
setBounds(400, 300, GAME_WIDTH, GAME_HEIGHT);
setVisible(true);
setResizable(false);
setTitle("TankWar");
setBackground(Color.green);
addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
new Thread(new PaintThread()).start();
}
private class PaintThread implements Runnable {
public void run() {
while(true) {
repaint();
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void main(String[] args) {
TankClient tc = new TankClient();
tc.launchFrame();
}}将原代码中的A句和B段去掉,在A处直接写Image offScreenImage = this.createImage(GAME_WIDTH, GAME_HEIGHT);
为何运行后小球就不向下走了,分析下是哪里的问题吧?
Image offScreenImage = this.createImage(GAME_WIDTH, GAME_HEIGHT);
是不起作用的,offScreenImage为null
作为成员初始化时,当前frame仍不可显示,所以返回值是null。