Question

学习head fierst java中在eclipse输入以下代码，出来的窗口里什么也没有，菜鸟求助。

import javax.swing.*;

import java.awt.*;

public class SimpleAnimation {

int x=70;
int y=70;

public static void main(String[] args) {
SimpleAnimation gui=new SimpleAnimation();
gui.go();
}

public void go(){
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

MyDrawPanel drawPanel = new MyDrawPanel();

frame.getContentPane().add(drawPanel);
frame.setSize(300, 300);
frame.setVisible(true);

for(int i=0;i<130;i++){
x++;
y++;

drawPanel.repaint();

try{
Thread.sleep(50);
}catch(Exception ex){}
}
}
class MyDrawPanel extends JPanel{
public void panitComponent(Graphics g){

g.setColor(Color.green);
g.fillOval(x, y,40,40);
}
}
}

Answer 1

drawPanel其实已经显示出来了，我给drawPanel设置了一个背景色，你可以看到效果。你没有调用panitComponent（）这个函数，也就是说没有画出来一个绿色的框框。
我改了两行你看看。如果对你有帮助，记得给好评O(∩_∩)O~
import javax.swing.*;

import java.awt.*;

public class SimpleAnimation {

int x=70;
int y=70;

public static void main(String[] args) {
SimpleAnimation gui=new SimpleAnimation();
gui.go();
}

public void go(){
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

MyDrawPanel drawPanel = new MyDrawPanel();
drawPanel.setBackground(Color.green);/** 为drawPanel设置背景色 */
frame.getContentPane().add(drawPanel);
frame.setSize(300, 300);
frame.setVisible(true);

for(int i=0;i<130;i++){
x++;
y++;

drawPanel.repaint();

try{
Thread.sleep(50);
}catch(Exception ex){}
}
}
class MyDrawPanel extends JPanel{
/** 加上这句话消除警告 */
private static final long serialVersionUID = 1L;

public void panitComponent(Graphics g){

g.setColor(Color.green);
g.fillOval(x, y,40,40);
}
}
}

追问

抱歉，回来晚了，在内部类里面填充了一个绿色的圆，再用上for循环，做出来应该是一个圆从左上角到右下角的一个小动画，但是出来没东西，只有窗口。

追答

/*MyDrawPanel这个类继承Applet类就ok了，记得给好评O(∩_∩)O~*/

import javax.swing.*;

import java.applet.Applet;
import java.awt.*;

public class SimpleAnimation {

int x=70;
int y=70;

public static void main(String[] args) {

SimpleAnimation gui=new SimpleAnimation();
gui.go();
}

public void go(){
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

MyDrawPanel drawPanel = new MyDrawPanel();
frame.getContentPane().add(drawPanel);
frame.setSize(300, 300);
frame.setVisible(true);

for(int i=0;i<130;i++){
x++;
y++;

drawPanel.repaint();

try{
Thread.sleep(50);
}catch(Exception ex){}
}
}
class MyDrawPanel extends Applet{
/** 加上这句话消除警告 */
private static final long serialVersionUID = 1L;
public void paint(Graphics g){
g.setColor(Color.green);
g.fillOval(x, y,40,40);
}
}
}