为什么paintComponent()方法总是在执行？

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;
}