为什么paintComponent()方法总是在执行？

YL_Show 2009-02-26 04:39:31

我仿照Java核心技术里面的一个程序，输入小时和分钟，在一个圆里面实现时针和分针。现在程序可以运行，但是在Debug的时候，由于我在画时钟的类的paintcomponent设置了一个断点，只要从代码切换到图形，就会停在这个地方。paintcomponent方法不是应该就在初始化的时候调用一次码，为什么总要执行。
下面是我的代码：
import java.awt.*;
import java.awt.geom.*;
import java.awt.geom.Ellipse2D.Double;
import javax.swing.event.*;

import javax.swing.*;

public class TextTest {
public static void main(String[] args) {
// Add a frame
TextTestFrame frame = new TextTestFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}

// Create a clock frame
class TextTestFrame extends JFrame {
public TextTestFrame() {
setTitle("TextTest");

// Draw the bottom line
JPanel panel = new JPanel();
JLabel label = new JLabel("Hours:");
panel.add(label);
hour_text = new JTextField("12", 3);
panel.add(hour_text);
JLabel label2 = new JLabel("Minutes:");
panel.add(label2);
minute_text = new JTextField("00", 3);
panel.add(minute_text);
add(panel, BorderLayout.SOUTH);

// Add the listener
clockFieldListener listener = new clockFieldListener();
hour_text.getDocument().addDocumentListener(listener);
minute_text.getDocument().addDocumentListener(listener);

// Draw clock panel
clockPanel = new DrawPanel();
add(clockPanel, BorderLayout.CENTER);
pack();
}

public void DrawClock() {
try {
clockPanel.setTime(Integer.parseInt(hour_text.getText()), Integer
.parseInt(minute_text.getText()));
clockPanel.repaint();
} catch (NumberFormatException e) {
//
}
}

public JTextField hour_text;
public JTextField minute_text;
public DrawPanel clockPanel;

public class clockFieldListener implements DocumentListener {
public void insertUpdate(DocumentEvent event) {
DrawClock();
}

public void changedUpdate(DocumentEvent event) {

}

public void removeUpdate(DocumentEvent event) {
DrawClock();
}
}
}

class DrawPanel extends JPanel {
public DrawPanel() {
setPreferredSize(new Dimension(2 * RADIUS + 1, 2 * RADIUS + 1));
}

public void paintComponent(Graphics g) { --就是这里了，Debug的时候只要切换出去就会又停在这里
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;

// Draw the circle
Ellipse2D elli = new Ellipse2D.Double();
elli.setFrameFromCenter(new Point2D.Double(RADIUS, RADIUS),
new Point2D.Double(0, 0));
g2.draw(elli);

// Draw the hour hand and minute hand
double hourAngle = Math.toRadians(90 - 360 * minute / (12 * 60));
DrawHand(g2, hourAngle, HOUR_HAND_LENGTH);

// Draw the minute hand
double minuteAngle = Math.toRadians(90 - 360 * minute / 60);
DrawHand(g2, minuteAngle, MINUTE_HAND_LENGTH);
}

public void DrawHand(Graphics2D g2, double angle, double length) {
Point2D p = new Point2D.Double(RADIUS + length * Math.cos(angle),
RADIUS - length * Math.sin(angle));
g2.draw(new Line2D.Double(p, new Point2D.Double(RADIUS, RADIUS)));
}

public void setTime(int h, int m) {
minute = h * 60 + m;
}

public static final int RADIUS = 100;
private double minute = 0;
private double HOUR_HAND_LENGTH = 0.6 * RADIUS;
private double MINUTE_HAND_LENGTH = 0.8 * RADIUS;
}

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