Question

Java paint方法为什么不能画出图像？

package test;

import java.awt.Frame;
import java.awt.Graphics;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

public class Frame01 extends Frame{

public void launchFrame(){
setSize(300, 300);
setLocation(100,100);
setVisible(true);
addWindowListener(new WindowAdapter() {

@Override
public void windowClosing(WindowEvent e) {
System.exit(0);
}

});
}

@Override
public void paint(Graphics g) {
g.drawLine(10,10,20 ,20 );
}

public static void main(String[] args) {
new Frame01().launchFrame();
}
}

为什么不能画出图像？

Answer 1

Frame是窗体，需要在其中添加 Jpanel 等组件。（标识符中用数字时最好用下划线连接，如 Frame_01）
public void launchFrame(){
setSize(300, 300);
setLocation(100,100);
this.add(new Panel_01());
setVisible(true);
addWindowListener(new WindowAdapter() {

@Override
public void windowClosing(WindowEvent e) {
System.exit(0);
}

});
}
class Panel_01 extends JPanel{
@Override
public void paint(Graphics g) {
g.drawLine(10,10,20 ,20 );
}
}