Question

java 红绿灯 为什么我用repaint（）图像就显示不出来

import java.awt.*;

import java.awt.event.*;
import javax.swing.*;
public class TrafficLight extends JFrame implements ActionListener {
private JRadioButton jrbRed,jrbYellow,jrbGreen;
private LightPanel light=new LightPanel();

public TrafficLight(){
getContentPane().setLayout(new BorderLayout());
JPanel jpRadioButtons=new JPanel();
jpRadioButtons.setLayout(new GridLayout(1,3));
jpRadioButtons.add(jrbRed=new JRadioButton("红灯"));
jpRadioButtons.add(jrbYellow=new JRadioButton("黄灯"));
jpRadioButtons.add(jrbGreen=new JRadioButton("绿灯"));

ButtonGroup group=new ButtonGroup();
group.add(jrbRed);
group.add(jrbYellow);
group.add(jrbGreen);

jrbRed.addActionListener(this);
jrbYellow.addActionListener(this);
jrbGreen.addActionListener(this);

getContentPane().add(jpRadioButtons,BorderLayout.SOUTH);
getContentPane().add(light);

}

@Override
public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub
//light.repaint();
if(e.getSource()==jrbRed){
light.ShowRed();
light.repaint();
}
if(e.getSource()==jrbYellow) {
light.ShowYellow();
light.repaint();
}

if(e.getSource()==jrbGreen){
light.ShowGreen();
light.repaint();
}

}

public static void main(String[]args){
TrafficLight frame=new TrafficLight();
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(500, 500);

}

}
class LightPanel extends JPanel{
private Graphics g;
protected void paintComponent(Graphics g){
super.paintComponent(g);
g.drawRect(getWidth()/2-25, getHeight()/2-65, 50, 140);
g.drawOval(getWidth()/2-20, getHeight()/2-60, 40, 40);
g.drawOval(getWidth()/2-20, getHeight()/2-15, 40, 40);
g.drawOval(getWidth()/2-20, getHeight()/2+30, 40, 40);

}
public void ShowRed(){
//repaint();
g=getGraphics();

g.setColor(Color.RED);
g.fillOval(getWidth()/2-20, getHeight()/2-60, 40, 40);
}
public void ShowYellow(){
g=getGraphics();
g.setColor(Color.yellow);
g.fillOval(getWidth()/2-20, getHeight()/2-15, 40, 40);
}
public void ShowGreen(){
g=getGraphics();
g.setColor(Color.green);
g.fillOval(getWidth()/2-20, getHeight()/2+30, 40, 40);
}
}

Answer 1

repaint()会调用paintComponent方法，而super.paintComponent(g);这句会清空组件

所以你的代码最后什么都不会变

给你改了一下，请参考

import java.awt.*;

import java.awt.event.*;
import javax.swing.*;
public class TrafficLight extends JFrame implements ActionListener {
	private JRadioButton jrbRed,jrbYellow,jrbGreen;
	private LightPanel light=new LightPanel();
	
	public TrafficLight(){
		getContentPane().setLayout(new BorderLayout());
		JPanel jpRadioButtons=new JPanel();
    	jpRadioButtons.setLayout(new GridLayout(1,3));
    	jpRadioButtons.add(jrbRed=new JRadioButton("红灯"));
    	jpRadioButtons.add(jrbYellow=new JRadioButton("黄灯"));
    	jpRadioButtons.add(jrbGreen=new JRadioButton("绿灯"));

    	ButtonGroup group=new ButtonGroup();
    	group.add(jrbRed);
    	group.add(jrbYellow);
    	group.add(jrbGreen);
   	
    	jrbRed.addActionListener(this);
    	jrbYellow.addActionListener(this);
    	jrbGreen.addActionListener(this);
   	
    	getContentPane().add(jpRadioButtons,BorderLayout.SOUTH);
    	getContentPane().add(light);
   	
	}

@Override
	public void actionPerformed(ActionEvent e) {
		// TODO Auto-generated method stub
		//light.repaint();
		if(e.getSource()==jrbRed){		
			light.ShowRed();
		}
		if(e.getSource()==jrbYellow)	{
			light.ShowYellow();
		}
			
		if(e.getSource()==jrbGreen){
			light.ShowGreen();
		}
		 
	}
	
	public static void main(String[]args){
		TrafficLight frame=new TrafficLight();
		frame.setVisible(true);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setSize(500, 500);
		
	}

}
 class LightPanel extends JPanel{
	int light_num = -1;
	Color[] colors = {Color.RED, Color.YELLOW, Color.GREEN};
	protected void paintComponent(Graphics g){
		super.paintComponent(g);
		g.drawRect(getWidth()/2-25, getHeight()/2-65, 50, 140);
		for(int i = 0; i < 3; i++) {
			if(i==light_num) {
				g.setColor(colors[i]);
				g.fillOval(getWidth()/2-20, getHeight()/2-60+i*45, 40, 40);
			}
			else {
				g.setColor(Color.BLACK);
				g.drawOval(getWidth()/2-20, getHeight()/2-60+i*45, 40, 40);
			}
		}
		 
	}
	public void ShowRed(){
		light_num = 0;
		repaint();
	}
	public void ShowYellow(){
		light_num = 1;
		repaint();
	}	 
	public void ShowGreen(){
		light_num = 2;
		repaint();
	} 
 }