Javanotes 5.1.2, Solution to Exercise 1, Chapter 6

Solution for Programmming Exercise 6.1

This page contains a sample solution to one of the exercises from Introduction to Programming Using Java.

Exercise 6.1:

In the SimpleStamperPanel example from Subsection 6.4.2, a rectangle or oval is drawn on the panel when the user clicks the mouse, except that when the user shift-clicks, the panel is cleared instead. Modify this class so that the modified version will continue to draw figures as the user drags the mouse. That is, the mouse will leave a trail of figures as the user drags the mouse. However, if the user shift-clicks, the panel should simply be cleared and no figures should be drawn even if the user drags the mouse after shift-clicking. Use your panel either in an applet or in a stand-alone application (or both). Here is an applet version of my solution:

The source code for the original panel class is SimpleStamperPanel.java. An applet that uses this class can be found in SimpleStamperApplet.java, and a main program that uses the panel in a frame is in SimpleStamper.java. See the discussion of dragging in Subsection 6.4.4. (Note that in the original version, I drew a black outline around each shape. In the modified version, I decided that it would look better to draw a gray outline instead.)

Discussion

In order to implement dragging in the new version, we need a MouseMotionListener in addition to the MouseListener that is already present in the original version. In the original, the panel class itself implements MouseListener, so I just added MouseMotionListener:

public class SimpleStamperPanelWithDrag extends JPanel 
                          implements MouseListener, MouseMotionListener { . . .

Of course, the mouse motion listener has to be registered with the panel in order for it to hear any events from the panel. This is done by adding the line

addMouseMotionListener(this);

to the constructor. Here "this" refers to the panel object itself and is used because the panel itself implements MouseMotionListener, and it will listen for mouse motion events from itself.

To finish the implementation of the mouse motion listener interface, the mouseMoved and mouseDragged methods must be added to the class. The program does not respond when the user moves the mouse without holding down any mouse button, so the mouseMoved method is empty. The mouseDragged method must draw a figure at the current mouse position; the code for this is almost identical to the existing drawing code in the mousePressed routine and can be copied from there. However, nothing should be drawn in the mouseDragged method if the user started the mouse drag gesture by shift-clicking. The discussion of dragging in Subsection 6.4.4, suggests that the program should use an instance variable named dragging to keep track of whether or not to draw anything in the mouseDragged method. In the mousePressed routine, this variable is set to false if the user shift-clicked, and to true otherwise. The mouseDragged routine checks the value of dragging; if the value is false, it means that the drag started with a shift-click and therefore nothing should be drawn.

The Solution

Here is the code for the modified panel class, with changes from the original (SimpleStamperPanel.java) shown in red (except for the change for black to gray for the outlines of the shapes):

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

/**
 * A simple demonstration of MouseEvents.  Shapes are drawn
 * on a black background when the user clicks the panel.  If the user
 * drags the mouse (except after a shift click, an additional shape is
 * drawn at each new position of the mouse).  If
 * the user Shift-clicks, the applet is cleared.  If the user
 * right-clicks the applet, a red rectangle is drawn.  Otherwise,
 * when the user clicks, a blue oval is drawn.  The contents of
 * the panel are not persistent.  For example, they might disappear 
 * if the panel is covered and uncovered.
 */
public class SimpleStamperPanelWithDrag extends JPanel 
                    implements MouseListener, MouseMotionListener {
   
   /**
    * This variable is set to true during a drag operation, unless the
    * user was holding down the shift key when the mouse was first
    * pressed (since in that case, the mouse gesture simply clears the
    * panel and no figures should be drawn if the user drags the mouse).
    */
   private boolean dragging;
   
   /**
    * This constructor simply sets the background color of the panel to be black
    * and sets the panel to listen for mouse events on itself.
    */
   public SimpleStamperPanelWithDrag() {
      setBackground(Color.BLACK);
      addMouseListener(this);
      addMouseMotionListener(this);
   }
   
   
   /**
    *  Since this panel has been set to listen for mouse events on itself, 
    *  this method will be called when the user clicks the mouse on the panel.
    *  This method is part of the MouseListener interface.
    */
   public void mousePressed(MouseEvent evt) {
      
      if ( evt.isShiftDown() ) {
            // The user was holding down the Shift key.  Just repaint the panel.  Since 
            // this class does not define a paintComponent() method, the method from the 
            // superclass, JPanel, is called.  That  method simply fills the panel with 
            // its background color, which is black.  The effect is to clear the panel.
         repaint();
         return;
      }
      
      dragging = true;  // If the shift was not down, start a drag operation during
                        // which additional shapes will be drawn as the mouse moves.
      
      int x = evt.getX();  // x-coordinate where user clicked.
      int y = evt.getY();  // y-coordinate where user clicked.
      
      Graphics g = getGraphics();  // Graphics context for drawing directly on this JPanel.
                                   // NOTE:  This is considered to be bad style!
      
      if ( evt.isMetaDown() ) {
            // User right-clicked at the point (x,y). Draw a blue oval centered 
            // at the point (x,y). (A gray outline around the oval will make it 
            // more distinct when ovals and rects overlap.)
         g.setColor(Color.BLUE);  // Blue interior.
         g.fillOval( x - 30, y - 15, 60, 30 );
         g.setColor(Color.GRAY); // Gray outline.
         g.drawOval( x - 30, y - 15, 60, 30 );
      }
      else {
            // User left-clicked (or middle-clicked) at (x,y). 
            // Draw a red rectangle centered at (x,y).
         g.setColor(Color.RED);   // Red interior.
         g.fillRect( x - 30, y - 15, 60, 30 );
         g.setColor(Color.GRAY); // Gray outline.
         g.drawRect( x - 30, y - 15, 60, 30 );
      }
      
      g.dispose();  // We are finished with the graphics context, so dispose of it.
      
   } // end mousePressed();
   
   
   /**
    * This method is called when the user drags the mouse while holding down
    * a mouse button.  If the mousePressed routine started a draw (rather
    * than simply erasing the panel), then a figure is added to the panel
    * at the current position of the mouse.
    */
   public void mouseDragged(MouseEvent evt) {
      if (dragging == false)
         return;
      int x = evt.getX();  // x-coordinate where user clicked.
      int y = evt.getY();  // y-coordinate where user clicked.
      Graphics g = getGraphics();  // Graphics context for drawing directly on this JPanel.
                                   // NOTE:  This is considered to be bad style!
      if ( evt.isMetaDown() ) {
            // The right mouse button is down. Draw a blue oval centered 
            // at the point (x,y). (A gray outline around the oval will make it 
            // more distinct when ovals and rects overlap.)
         g.setColor(Color.BLUE);  // Blue interior.
         g.fillOval( x - 30, y - 15, 60, 30 );
         g.setColor(Color.GRAY); // Gray outline.
         g.drawOval( x - 30, y - 15, 60, 30 );
      }
      else {
            // Draw a red rectangle centered at (x,y).
         g.setColor(Color.RED);   // Red interior.
         g.fillRect( x - 30, y - 15, 60, 30 );
         g.setColor(Color.GRAY); // Gray outline.
         g.drawRect( x - 30, y - 15, 60, 30 );
      }
   }
   

   /**
    * When the user released the mouse, dragging is set equal to false.
    */
   public void mouseReleased(MouseEvent evt) { 
      dragging = false;
   }

   
   // The next three empty routines are required by the MouseListener interface.
   // Since they don't do anything in this class, so their definitions are empty.
   
   public void mouseEntered(MouseEvent evt) { }
   public void mouseExited(MouseEvent evt) { }
   public void mouseClicked(MouseEvent evt) { }
   
   // The next empty routine is required by the MouseMotionListener interface.
   
   public void mouseMoved(MouseEvent evt) { }
   
} // end class SimpleStamperPanelWithDrag

In order to use this panel, it must be placed in either a JFrame or a JApplet. Here is the source code for a main program that uses the JFrame version. This is a very simple modification of the original version, SimpleStamper.java:

import javax.swing.JFrame;

/**
 * A stand-alone application that opens a window containing a
 * SimpleStamperPanelWithDrag.  When the user clicks the panel, a red
 * rectangle is added to the window.  When the user right-clicks,
 * a blue oval is added.  When the user shift-clicks, the drawing
 * is cleared.  If the user drags the mouse (except after a shift-click),
 * another figure is drawn at each new position of the mouse.
 * The program ends when the user closes the window by clicking its close box. 
 */
public class SimpleStamperWithDrag {

   public static void main(String[] args) {
         JFrame window = new JFrame("Simple Stamper With Drag");
         SimpleStamperPanelWithDrag content = new SimpleStamperPanelWithDrag();
         window.setContentPane(content);
         window.setSize(350,250);
         window.setLocation(100,100);
         window.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
         window.setVisible(true);
      }

}

And here is the source code for an applet that uses the modified version. This is a very simple modification of the original version, SimpleStamperApplet.java:

import javax.swing.JApplet;

/**
 * An applet that shows a SimpleStamperPanelWitheDrag.  When the user clicks the 
 * panel, a red rectangle is added to the window.  When the user right-clicks,
 * a blue oval is added.  When the user shift-clicks, the drawing is cleared. 
 * If the user drags the mouse (except after shift-clicking), another figure
 * is drawn at each new position of the mouse.
 */
public class SimpleStamperAppletWithDrag extends JApplet {

   public void init() {
      SimpleStamperPanelWithDrag content = new SimpleStamperPanelWithDrag();
      setContentPane(content);
   }
}