Getting Started with Visual J++

In this course, you will be creating Java applets, which are programs that can be run over the World Wide Web. A Java applet requires a web page in order to view it. And, in order for everyone to see your applet, both the web page and applet code must be saved in your public_html directory within your acpub account. In this course, we will put all of our Java programs in separate directories within the course directory, cps4, you created earlier within your public_html directory. Each time you transfer your work into your cps4 directory, using FTP, it is important that you create a separate directory for each assignment or you will eventually make a mistake and copy over your previous work.

Additionally, it is important that you create your own temporary working space on the classroom machines. For example, before starting each exercise, create a directory whose name is the combination of you and your partners acpub IDs within the local directory C:\classes\cps4. Without such a system, you may lose your work because you saved over someone else's work or transfer the wrong file (that had the same name, but different content).

Projects

Once you have selected a kind of project, you must name your project and pick some place to save it on the classroom machine. For this exercise, please use the project name Picture and save it in the directory C:\classes\cps004\\Picture. Once you have filled these values into the bottom section of the New Project panel, click Open to create the project.

When you open a GP Applet project, you should find the following code in the file Applet.java. Every program you write this semester will include at least this code since it is the starting point of your Java programs.

     /*
      * Authors: YOUR NAME HERE
      *          YOUR PARTNER'S NAME HERE
      * Date:    TODAY'S DATE
      * Course:  CPS 4
      * Purpose: Practice Java
      */
     public class Applet extends GP.Containers.Applet
     {
         public Applet ()
         {
         }
     }

Editing Java Code

For example, you should note that the code shown above is highlighted with colors when viewed in the Visual J++ editor (blue words are keywords in Java, meaning you cannot name anything in your code with the same name, and light green words are comments, meaning they are ignored within your program). Additionally, the editor will attempt to indent your code properly so that it is easier to read. However, unlike word processors, none of this information is stored when the code is saved because it is only used to help make your coding easier --- not to present your code on the web or in a book.

You should now edit the code shown above, that appears in the file Applet.java within your project to resemble the code shown below. Be careful that you type it just as it appears because the punctuation and case is very important in Java. You will notice sometimes after you type return to end a line, the cursor will automatically move to the next line properly lined up or indented. This is a feature your should get used to using because when it does not move to the correct position on the next line, the editor is trying to let you know you have not typed something correctly above that line.

     /*
      * Authors: Robert Duvall
      * Date:    July 9, 2001
      * Course:  CPS 4
      * Purpose: Practice Java
      */
     public class Applet extends GP.Containers.Applet
     {
         public Applet ()
         {
             new GP.Shapes.Oval();
         }
     }

Until you save your file, it is stored in a temporary buffer (the region in Visual J++ in which you are typing). You should save your project often to ensure none of your changes are lost inadvertently. You will not be granted any leniency if you lose your work because you did not save it properly. You can save your project by selecting "Save All" from the file menu. Note, you chose where to save your project when you created it at the beginning of the tutorial. Everything you work on within the project will be saved in that folder, so you should not attempt to save individual files separately.

Building a Project

If there is a syntax error in the program, a message will appear in the Task window at the bottom of the environment. To find the error within your code, simply double-click on the error with the left mouse button. This will take your editor's cursor to the line on which the error appears, so you can use it to cycle through multiple errors, fix them, and then recompile. If there are multiple error messages, fix the first one only and then recompile. Often one error causes the compiler to list additional errors that are based solely on the first error.

An example of an error message is:

     Undefined name 'GP.Shape.Oval'

Also note that while you are typing, most syntax errors are underlined with a red squiggly line, much like spelling errors are highlighted in modern word processors. If you hold your mouse over the underlined code, a small balloon box should appear describing the guess as to the problem. This can help you fix your errors even before compiling! If you do not understand the error message, talk to one of the course staff.

When the program has been successfully compiled continue with the next section. The compiled code is automatically saved in a file called Applet.class In general, a file with a .class extension is created for each Java class you have in your project (you can verify this by opening your project folder using Windows Explorer). The compiled code is important when running your program over the WWW, but it can always be recreated from your original source code by building your project again.

Viewing your Project

In order to view your program, you should click on the first option from the Debug menu, Start. This should start a browser for you pointed at the index.html web page that is part of your project. It may take a few seconds for your applet to appear, but be patient. You can watch the status information at the bottom of the browser window to verify its progress. If the browser does not start automatically, a dialog box will appear asking you to verify that you want to view the results using your web page. In this case, select index.html from the choice box in the middle of the dialog and click the Okay button.

If you have done everything correctly, you should see the following: Once you have seen that your applet does what you expect, you need to close the browser to return to the Visual J++ environment. Do this by clicking on the "X" in the top right corner of the browser or by selecting the End option from the Debug menu.

Changing Shapes Attributes

     /*
      * Authors: Robert Duvall
      * Date:    July 9, 2001
      * Course:  CPS 4
      * Purpose: Practice Java
      */
     public class Applet extends GP.Containers.Applet
     {
         public Applet ()
         {
             GP.Shapes.Oval oval = new GP.Shapes.Oval();
         }
     }

Since most everything in Java is an object, we must also create an object to represent the attribute we want to change about the shape. Likewise, we must name it so that it can be referred to later. Putting it all together, for example, you can set the position of the oval by calling its SetPosition function and passing it a GP.Attributes.Coordinate object after you have created it using new, like this:

   /*
    * Authors: Robert Duvall
    * Date:    July 9, 2001
    * Course:  CPS 4
    * Purpose: Practice Java
    */
   public class Applet extends GP.Containers.Applet
   {
       public Applet ()
       {
           GP.Shapes.Oval oval = new GP.Shapes.Oval();
           GP.Attributes.Coordinate center = new GP.Attributes.Coordinate(200, 200);
           oval.SetPosition(center);
       }
   }

You can find a list of shapes and their attributes here.