一段在jcomponent上绘制矩形框的代码，很不错的代码。

/* From http://java.sun.com/docs/books/tutorial/index.html */
/*
 * SelectionDemo.java is a 1.4 application that requires one other file:
 * images/starfield.gif
 */

import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.event.MouseInputAdapter;
import java.awt.*;
import java.awt.event.MouseEvent;

/*
 * This displays an image. When the user drags within the image, this program
 * displays a rectangle and a string indicating the bounds of the rectangle.
 */
public class SelectionDemo {
  JLabel label;

  static String starFile = "starfield.gif";

  private void buildUI(Container container, ImageIcon image) {
    container.setLayout(new BoxLayout(container, BoxLayout.PAGE_AXIS));

    SelectionArea area = new SelectionArea(image, this);
    container.add(area);

    label = new JLabel("Drag within the image.");
    label.setLabelFor(area);
    container.add(label);

    //Align the left edges of the components.
    area.setAlignmentX(Component.LEFT_ALIGNMENT);
    label.setAlignmentX(Component.LEFT_ALIGNMENT); //redundant
  }

  public void updateLabel(Rectangle rect) {
    int width = rect.width;
    int height = rect.height;

    //Make the coordinates look OK if a dimension is 0.
    if (width == 0) {
      width = 1;
    }
    if (height == 0) {
      height = 1;
    }

    label.setText("Rectangle goes from (" + rect.x + ", " + rect.y
        + ") to (" + (rect.x + width - 1) + ", "
        + (rect.y + height - 1) + ").");
  }

  /** Returns an ImageIcon, or null if the path was invalid. */
  protected static ImageIcon createImageIcon(String path) {
    java.net.URL imgURL = SelectionDemo.class.getResource(path);
    if (imgURL != null) {
      return new ImageIcon(imgURL);
    } else {
      System.err.println("Couldn't find file: " + path);
      return null;
    }
  }

  /**
   * Create the GUI and show it. For thread safety, this method should be
   * invoked from the event-dispatching thread.
   */
  private static void createAndShowGUI() {
    //Make sure we have nice window decorations.
    JFrame.setDefaultLookAndFeelDecorated(true);

    //Create and set up the window.
    JFrame frame = new JFrame("SelectionDemo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    //Set up the content pane.
    SelectionDemo controller = new SelectionDemo();
    controller.buildUI(frame.getContentPane(), createImageIcon(starFile));

    //Display the window.
    frame.pack();
    frame.setVisible(true);
  }

  public static void main(String[] args) {
    //Schedule a job for the event-dispatching thread:
    //creating and showing this application's GUI.
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        createAndShowGUI();
      }
    });
  }

  private class SelectionArea extends JLabel {
    Rectangle currentRect = null;

    Rectangle rectToDraw = null;

    Rectangle previousRectDrawn = new Rectangle();

    SelectionDemo controller;

    public SelectionArea(ImageIcon image, SelectionDemo controller) {
      super(image); //This component displays an image.
      this.controller = controller;
      setOpaque(true);
      setMinimumSize(new Dimension(10, 10)); //don't hog space

      MyListener myListener = new MyListener();
      addMouseListener(myListener);
      addMouseMotionListener(myListener);
    }

    private class MyListener extends MouseInputAdapter {
      public void mousePressed(MouseEvent e) {
        int x = e.getX();
        int y = e.getY();
        currentRect = new Rectangle(x, y, 0, 0);
        updateDrawableRect(getWidth(), getHeight());
        repaint();
      }

      public void mouseDragged(MouseEvent e) {
        updateSize(e);
      }

      public void mouseReleased(MouseEvent e) {
        updateSize(e);

 //在这里矩形框绘制完成后可以做很多应用
      }

      /*
       * Update the size of the current rectangle and call repaint.
       * Because currentRect always has the same origin, translate it if
       * the width or height is negative.
       *
       * For efficiency (though that isn't an issue for this program),
       * specify the painting region using arguments to the repaint()
       * call.
       * 
       */
      void updateSize(MouseEvent e) {
        int x = e.getX();
        int y = e.getY();
        currentRect.setSize(x - currentRect.x, y - currentRect.y);
        updateDrawableRect(getWidth(), getHeight());
        Rectangle totalRepaint = rectToDraw.union(previousRectDrawn);
        repaint(totalRepaint.x, totalRepaint.y, totalRepaint.width,
            totalRepaint.height);
      }
    }

    protected void paintComponent(Graphics g) {
      super.paintComponent(g); //paints the background and image

      //If currentRect exists, paint a box on top.
      if (currentRect != null) {
        //Draw a rectangle on top of the image.
        g.setXORMode(Color.white); //Color of line varies
        //depending on image colors
        g.drawRect(rectToDraw.x, rectToDraw.y, rectToDraw.width - 1,
            rectToDraw.height - 1);

        controller.updateLabel(rectToDraw);
      }
    }

    private void updateDrawableRect(int compWidth, int compHeight) {
      int x = currentRect.x;
      int y = currentRect.y;
      int width = currentRect.width;
      int height = currentRect.height;

      //Make the width and height positive, if necessary.
      if (width < 0) {
        width = 0 - width;
        x = x - width + 1;
        if (x < 0) {
          width += x;
          x = 0;
        }
      }
      if (height < 0) {
        height = 0 - height;
        y = y - height + 1;
        if (y < 0) {
          height += y;
          y = 0;
        }
      }

      //The rectangle shouldn't extend past the drawing area.
      if ((x + width) > compWidth) {
        width = compWidth - x;
      }
      if ((y + height) > compHeight) {
        height = compHeight - y;
      }

      //Update rectToDraw after saving old value.
      if (rectToDraw != null) {
        previousRectDrawn.setBounds(rectToDraw.x, rectToDraw.y,
            rectToDraw.width, rectToDraw.height);
        rectToDraw.setBounds(x, y, width, height);
      } else {
        rectToDraw = new Rectangle(x, y, width, height);
      }
    }
  }
}