主观题：绘制红绿灯

绘制红绿灯，包括红黄蓝三盏灯，初始时各盏灯为灰色。点击启动按钮，则各盏灯交替点亮，即先为最上方的红色灯，然后是中间的黄色灯，最后是下方的绿色灯，时间间隔可以设为3秒，循环交替。点击停止按钮，则各盏灯变为灰色。示例界面如下图所示。


提示：可以使用Timer设定时间间隔，定时重绘界面。






答案：import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Ellipse2D;

import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

public class ColorChange {
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
TrafficLightFrame frame = new TrafficLightFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
});
}
}

class TrafficLightFrame extends JFrame {
private Timer timer;
private TrafficLightComponent lightComponent;

public TrafficLightFrame() {
setSize(220, 250);
setTitle("交通灯");
timer = new Timer(1000, new ChangeColorAction());

lightComponent = new TrafficLightComponent();
add(lightComponent, BorderLayout.CENTER);
JButton startButton = new JButton("启动");
startButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
timer.start();
}
});
JButton stopButton = new JButton("停止");
stopButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
timer.stop();
lightComponent.setColorNum(3);
repaint();
}
});

JPanel buttonPanel = new JPanel();
buttonPanel.add(startButton);
buttonPanel.add(stopButton);
add(buttonPanel, BorderLayout.SOUTH);
}

class ChangeColorAction implements ActionListener {
public void actionPerformed(ActionEvent event) {
lightComponent.setColorNum((lightComponent.getColorNum() + 1) % 3);
repaint();
}
}

}

class TrafficLightComponent extends JComponent {
private int colorNum = 3;

public int getColorNum() {
return colorNum;
}

public void setColorNum(int colorNum) {
this.colorNum = colorNum;
}

public void paintComponent(Graphics g) {
Graphics2D g2 = (Graphics2D) g;

Ellipse2D redLight = new Ellipse2D.Double(80, 0, 40, 40);
Ellipse2D yellowLight = new Ellipse2D.Double(80, 50, 40, 40);
Ellipse2D greenLight = new Ellipse2D.Double(80, 100, 40, 40);

switch (colorNum) {
case 1:
g2.draw(yellowLight);
g2.draw(greenLight);
g2.setPaint(Color.red);
g2.fill(redLight);
break;
case 2:
g2.draw(redLight);
g2.draw(greenLight);
g2.setPaint(Color.yellow);
g2.fill(yellowLight);
break;
case 0:
g2.draw(yellowLight);
g2.draw(redLight);
g2.setPaint(Color.green);
g2.fill(greenLight);
break;
case 3:
g2.draw(redLight);
g2.draw(yellowLight);
g2.draw(greenLight);
break;
}
}
}