A Useful Java Programming Environment

Working on Java Programs

When you are working on a program in any object-oriented language, you are typically working with several files, each containing code for a class. You do not write all of the code at once, compile it, run it, and expect to be done. Instead, you add a little bit of functionality at a time. For each increment of functionality, you usually need to modify several of the class files, compile them, then run the modified program to check the correctness of the new functionality. Even if you are an experienced programmer, you should expect to make some mistakes, so you may need to go back and modify some of the files, recompile them, and rerun the program. For this kind of work, it is important to have program development tools set up to simplify the editing, compilation, and program execution.

For CS 2121, I strongly urge you to use the emacs editor for creating and modifying program files. It is set up so that it runs in its own window, independent of other development software that you are using.

For most of the programs that you will write, you will be creating Java applets. The most convenient tool for executing applets is the appletviewer program. Again, it runs in its own window.

Finally, you will be writing programs on a UNIX machine, so you need terminal window for giving UNIX commands. This is the window that is used to compile your programs using the javac compiler. When you are working from a Windows machine, such as the machines in the MWAH 177 lab, this is the first window that you open up with X-Win32. On UMD Windows machines, you can start up an X-Win32 terminal to our "ub" machine by doing the following.

• Click on the "Start" button.
• In the "Start" menu, click on the "Programs" menu item.
• In the "Programs" menu, click on the "UMDNET Communications" menu item.
• In the "UMDNET Communications" menu, click on the "X-Win32 5.1" menu item.
• In the "X-Win32 5.1" menu, click on the "X-Win32" menu item.
• Enter your UMD login name and UMD password when prompted.

On UNIX machines, such as the ones in the Computer Science Lab (HH 314), you can open up a terminal window directly.

Most of the time you have three windows for doing your work.

• a terminal window for compiling and other UNIX commands
• an emacs window for editing program files
• an appletviewer window for executing your applets

It is helpful to set up separate directories for each of the exercises and assignments that you work on. The sections below assume that you are doing this.

The Terminal Window

This window is used for all UNIX commands. The commands that you use depend on how far along you are in developing your program.

When you are first starting an exercise or assignment,

• mkdir directory
  Creates a new directory for the exercise or assignment. You can use whatever name you want for the new directory.
• cd directory
  Moves you into the new directory. The name that you specify for the directory should be the same as in the mkdir command.
• emacs &
  Starts up the emacs editing window. The ampersand (&) tells the UNIX command interpreter to start the emacs program, but not to wait for it to finish. This allows you to give UNIX commands without having to stop the editor. The ampersand is often used with graphical user interface programs that operate in their own window.

When you are starting a new work session after the first, you will normally use the same commands except for the mkdir command. You only need to create the directory the first time.

After you have done some editing and are ready to compile your program,

• javac java-file
  Compiles the class in the specified file. Its name must end with a .java suffix. You can specify more than one java file in the command. Some people (myself included) use the following variation.
• javac *.java
  This compiles all of the java files in your current working directory.

When you are ready to run your program for the first time in a work session,

• appletviewer html-file &
  This starts up the appletviewer program. The HTML file tells the appletviewer program which class file it is working with and the size of the display area that it needs. It is usually the first file that you create for a program. Once again, the ampersand just starts up the appletviewer window, allowing you to give further commands in the terminal window.

For further work in a work session, if you use emacs and appletviewer as described in the following sections then you will only need to give their commands once in a work session. Then further work involves cycles of editing in the emacs window, giving the javac command in the terminal window to compile, and reloading the program in the appletviewer window.

Here are some more UNIX file commands that you may use from time to time.

• rm file
  Removes the specified file. You can use this command to clean up files after you have completed an exercise or assignment. Also see the rmdir command below.
• cp file another-file
  Copies the first specified file to another file. If necessary, a new file is created for the second file. You can use this command to create a new HTML file for an exercise or assignment. After creating a new directory for the work, but before moving into the new directory, copy the HTML file from an earlier exercice or assignment into your new directory. When you are naming a file that is not in your current working directory you need to prefix the name with the name of the directory followed by a slash (/). There should not be any spaces in a file or directory name.
• mv file new-name
  The mv command just renames a file or a directory. You may want to use this command if you misspell the name that you specified in emacs.

Here are some UNIX directory commands that you may occasionally use.

• ls
  Lists the files in your current working directory.
• rmdir directory
  Removes the specified directory. Its files must already have been removed with the rm command. You should move into the directory that contains the one you want to remove before giving this command. You will only need this command after you have completed an exercise or assignment. I recommend that you do not remove your files for an exercise or assignment until you are sure a correct grade has been recorded for it.

UNIX maintains a list of the 20 most recent commands. You can re-execute an earlier command with just an exclamation point (!) followed imediately by enough of the command to distinguish it from other commands. If you earlier recompiled everything with

    javac *.java

    !j

The emacs Window

If you have started up the emacs program with "emacs &" then you will not need to shut it down to compile or start the appletviewer program. You can just keep the emacs window open for the duration of a work session. Each time you open a file with the "Open File" menu item on the "Files" menu, it will be added to the emacs buffer list. You can then re-edit it by selecting it from the "Buffers" menu. Be sure to save your work on one file before recompiling or moving on to another file. This is done with the "Save Buffer" menu item on the "Files" menu.

The appletviewer Window

If you have started up the appletviewer program with "appletviewer &" then you will not need to shut it down to do other work. After you have modified your program and recompiled it, you can have appletviewer reload the revised program by selecting "Reload" from the "Applet" menu.

Be sure that you have saved files and recompiled them before reloading the program. If you have made changes in a program but do not get any expected changes in its behavior in appletviewer, it may be due to failure to save the buffer or recompile. Even if you think that you saved changes and recompiled, do it again to make sure.