Getting started with Java programming in BlueJ

CPS313 Handout

18 August 2006

Abstract:

Setting up your Java environment

• The Java Development Kit (JDK) 1.5.0 is installed on all PCs in the labs. But if you would like to install it at home, then you have to downlowd the latest update of JDK 1.5.0 from Sun (you do not need to download NetBeans and Java EE). Note that it will be better for you to download JDK 1.5.0 (not the JDK 1.6.0 release of Java). You can also install JDK 1.5.0 provided on every CD shipped with the textbook. Once you have downloaded and installed JDK 1.5.0 on your computer, you will have to update the PATH environment variable (the name of this variable can be Path on your computer). If you would like to see the value of your current PATH environment variable type the command path in the command prompt. This command shows what your current PATH is. Alternatively, you can type echo %PATH% to see what the PATH variable is or type set to see all environment variables. There are cases when you would like to make permanent additions to the PATH. For example, after installing Java, you should add the directory for the Java commands to the PATH. In Windows XP, open the Control Panel. Make sure it is in classic mode. Click the System icon, then on the Advanced tab, then on the Environment Variables button (see the figure below).

  

  Edit the PATH variable in the System Variables section (you might need to edit PATH both in System Variables, and in User Variables). The same method works for Windows NT and Windows 2000. Of course, you can do this only on your home computer (only the system administrators can edit the system PATH variable on the computers in the labs). If you installed the recent release of Java 1.5.0, you will probably add

  C:\Program Files\Java\jdk1.5.0_08
  

  (or something similar) to your PATH variable. Be very careful with the spacing when you edit the PATH variable. There must be no spaces around the symbols   =   ;   %  

• Setting your CLASSPATH. You have to tell Java where the system libraries are located. Your programs will normally use these libraries (they have the extension .jar). If the Java compiler will not know where to find them, then you will be able to run only programs of very limited functionality. But we would like to study Java using the robot libraries provided with the textbook. For this reason, we have to make sure that these libraries are accessible to the Java compiler. To find the current CLASSPATH environment variable, open Command Prompt and type echo %CLASSPATH%   or symply type set to see all environment variables. If your machine is running Windows XP, then you can set globally the CLASSPATH variable (in the control panel) following the same sequence of steps as described above for PATH. If CLASSPATH is not listed in the User Variables section, then you need to create a new entry by clicking "New" and then filling in the two entries in the pop-up, fairly obviously the Variable Name is CLASSPATH and the Variable value is similar to

  C:\Program Files\Java\jdk1.5.0_08\lib\tools.jar;C:\UserName\cps313\Robots\becker.jar;. 
  

  (Of course, UserName should be your name, the path to the Robots can be different on your machine depending on where your installed libraries from the CD provided with the textbook). The trailing "dot" is important: it tells to Java compiler to use classes (if there are any) from the current directory. If the CLASSPATH is already listed, then you can append the path by clicking on "Edit" and simply tacking the new setting on the end of whatever is already there (remember to add a semicolon first). After making your changes, click on the OK button on both the Environment Variables dialog and the System dialog. You will not need to restart your computer, but you will need to restart any applications which may have read the CLASSPATH settings before your changes (including any Command Prompt windows).

  If your machine is running Linux you can test whether your CLASSPATH is already correct with the following command:

  echo $CLASSPATH
  

  The output should be similar to one of these /home/yourUserName/313/Robots:/sw1/jdk1.5.0_06/lib/:.     /home/yourUserName/313/Robots:. If this is not the case, open the file called ".bashrc" in your home directory with a text editor, and add the following line:

  export CLASSPATH=/home/yourUserName/313/Robots:.
  

• If you had to set your PATH and/or your CLASSPATH, log out and log in again to make sure the changes takes effect.

BlueJ

Typing in your program

/home/yourname/homework/

c:\homework\

• Choose Project->New Project from the menu. In the file dialog, navigate to the parent directory that holds your projects (such as homework). In the name field, enter the name of your new project (for example, hw1).

  

• Click Edit New Class, type "Hello" in the "Class Name" field, and click Ok.

  

• Right click on the "Hello" class, and select "Open Editor". The editor window contains a template of the class. Clear out the sample instance field and the sample method. Insert the code

  	public static void main(String[] args)
  	{
  		System.out.println("Hello World!");
  	}
  

  in the class, and save the file (Class Save).

  

• You will get this file: Hello.java

Compile and run your program

• Compile your program by clicking the "Compile" button. Or by right-clicking on the class in the GUI, or by selecting "Compile" in the "tools" menu. When the "Hello" class is compiled, it is no longer striped:

  

• Run your program by right-clicking the "Hello" class again. Now that is has compiled, a few new menu items has appeared. You can start a new instance of the "Hello" class, and inspect it's variables etc. This is good for testing and debugging. You can also run the "main" function:

  

  BlueJ will now display your output ("Hello World!") in a terminal window. To close terminal window, click Options --> Close. Once you learnt how to write new Java programs, you can explore how to run example programs from the textbook.

  How to work with Java programs from the textbook.

  It is assumed below that you installed all examples in the Robots folder as specified on the Web site and now you have sub-folders "becker", "doc", "examples" in this folder. If you decided to use folder with another name, the instructions below should apply to that folder.

  

  Before you start, customize BlueJ settings. Click Tools-->Preferences-->Libraries. Add path to each library that BlueJ will need to use, e.g.:

  C:\UserName\cps313\Robots\becker.jar
  

  In the sequel, it is assumed that you have updated the CLASSPATH environment variable as specified above. If you are not sure about CLASSPATH, talk to a TA. For simplicity, in this handout I will discuss only the very first program from the textbook:   DeliverParcel.java   but you can run all other programs following similar instructions.

  1. Start   bluej   program.
  2. Click on Project --> Open Non BlueJ. You will see all your folders. navigate to the folder Robots and click on it, and then navigate to Robots --> examples --> ch01 --> deliverParcel. Do not go inside this folder.

     

     Click on "Open in BlueJ" button near the bottom-right corner. A new Project window will open.

     

  3. Click on the yellow rectangle that represents DeleverParcel class. Compile it, either by clicking on Compile, or by right-clicking on the rectangle and choosing compile in the pull-down menu. Wait, until you will see "Compiling.. Done" at the bottom-left corner of BlueJ.
  4. Right-click on the yellow DeleverParcel rectangle and choose "main" in the pull-down menu. You will see a new window "BlueJ: Method Call". Click "OK".

     

  5. A new widow with the simulated robots world will open. Use controls in the window. Exit when you are done.

     

  6. Note that if you decide to work with DeleverParcel.java project again, you will have to use Project --> Open Project menu, because once BlueJ opened a foreign project, it creates a native project to support possible future modifications.

  A convenient text editor

  If you are looking for a convenient text editor that facilitates development of Java programs, use jedit

  

  It is free, it is written in java, and it works on all major platforms. You can download a personal copy from http://www.jedit.org/

  Bibliography

  1

  The j2sdk documentation. Java 2 Standard Edition 5.0 API Specification, http://java.sun.com/docs/index.html.

  2

  BlueJ Homepage. http://www.bluej.org/

  3

  Jedit Homepage. http://www.jedit.org/

  
  

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  Acknowledgments.
  This handout is based on "Exercise 10: Getting started with Java programming in Linux" prepared by Hanne Munkholm (IT University of Copenhagen) and on Cay Horstmann's handout on BlueJ. Part of this document was generated using the L aTeX2HTML translator Version 2002 (1.62) and edited manually.