CS 3291 (Java with Internet Applications):
Homework #5

Assigned:

????

Due:

????

Credit:

10 points.

Objective:

(1) Modify a program that uses the AWT classes to work with graphical objects, and (2) convert an application to an applet.

Description:

For this program, you are given a stand-alone Java application program. You are to make one enhancement to its behavior (as described) and convert it to an applet.

In its current form, the program displays:

• a Canvas-type object on which the user can draw filled polygons (by clicking the left mouse button to define three or more points and then the right mouse button to "finish" the polygon (connect the last point to the first),
• a Choice object whose items correspond to colors, and whose state governs the color of the polygon being defined, and
• a "clear" button that clears the display.

What you are to add is:

• A Label showing how many completed polygons are currently defined (not including the one that is "in work" -- being defined but not "finished" yet).

See the "before" and "after" pictures in the Detailed description below.

You are also to convert the program into an applet, to be executed via the command:

appletviewer PolyDrawApplet.html

(You will be given the HTML file PolyDrawApplet.html.)

Instructions:

1. Review the Detailed description and Helpful hints sections below.
2. Download the initial program PolyDraw.java. Also download CloseableFrame.java (you will not need this class for your applet, but will need it to run the initial program).
3. Download the required HTML file PolyDrawApplet.html. (Note that to display or download this file you will probably need to use your browser's "display source" option.)
4. Modify the program as needed, producing a PolyDrawApplet class (in a PolyDrawApplet.java file).
5. Test the program until you are satisfied that it works.
6. Submit your Java source file, as described in How to submit homework. You need only submit PolyDrawApplet.java, not PolyDrawApplet.html.

Helpful hints:

• To convert a stand-alone application to an applet, you need to do the following:
  • Make the "main" class a subclass of java.applet.Applet. Note that appletviewer requires that the class you start be declared public, as must its constructor(s). You can probably also remove main(), since it's not needed for an applet.
  • Override the init() method, and move setup work from the main class's constructor to init(). You may be able to dispense with the constructor altogether. You can also remove calls to pack() and show(), since the applet viewer or browser will take care of sizing the window and putting it on the screen.
• The initial program should be correct. However, mouse-click events sometimes appear to get lost if the user moves too fast. This may be a problem with the JVM implementation; a workaround is just to slow down a little.
• If your applet fails with an IllegalAccessError exception, try removing the "private" designation from the "pad" variable. There is a bug in some releases/versions of the JDK that produces such exceptions when you try to access a private variable from an inner class in an applet (which presumably should work).

Detailed description:

Current user interface:

The initial program's interface looks like this (under Unix/X -- should look similar though maybe not identical under Windows). (Here, the user has drawn one polygon, the black triangle, and is in the process of drawing a second polygon in blue.)

Desired user interface:

Your applet's interface should look like this (using the JDK appletviewer under Unix/X -- should look similar though maybe not identical under Windows). (As in the previous illustration, here the user has drawn one polygon, the black triangle, and is in the process of drawing a second polygon in blue.)

The value displayed in the label at the top of the panel should reflect the total number of "completed" polygons -- that is, it should change when (1) the user clicks the right mouse button to "finish" a polygon, and (2) when the user presses the clear button to reset everything.

Grading:

This homework is worth 10 points:

• 8 points for correctness. I will test your program with a standard test "script" (not automated this time, however) and verify that it behaves as described.
• 2 points for style. See Programming style for notes on what I consider good programming style. Here, I will be continuing to look for good object-oriented design, which in this case means making the required changes without too much disruption to the existing program.