Creating and Using Java Packages and Jar Files

Part A. Packages

• A package is a collection of related classes.

• A class path is a list of folders in which the Java compiler and interpreter look to find imported classes.

• To create and test a package named StackClass, follow these directions:
  1. Reread the document Running Java from the DOS Prompt.

  2. Make sure that the path to the Java compiler and interpreter is set up correctly.

  3. Create the new folder c:\mypackages\stackqueue.

  4. Download the source code files Stack.java and Queue.java into c:\mypackages\stackqueue.
     Notice that the files Stack.java and Queue.java have as their first line
     package stackqueue;

  5. Compile the downloaded source code files from Step 2 to create Stack.class and Queue.class in c:\mypackages\stackqueue. You can use BlueJ, TextPad, or compile directly from the DOS prompt.

  6. Create the folder c:\testfolder.

  7. Download the source code file TestStackQueue.java into c:\testfolder.
     Notice that Main.java has as its first line
     import stackclass;

  8. Compile TestStackQueue.java with:
     javac -classpath c:\mypackages Main.java
     This creates TestStackQueue.class.

  9. Run TestStackQueue.class with:
     java -classpath c:\mypackages Main

  10. Alternatively, you can set the classpath environment variable with set classpath=.;c:\mypackages

  11. Then you can compile and run TestStackQueue with these lines:
      javac TestStackQueue.java
      java TestStackQueue

Part B. Jar Files

• A Jar file is compressed file, similar to a zip file, containing the .class files of a package.

• To import a Jar file instead of the uncompressed .class files, following these directions:
  1. Open a Command Prompt window (Start, All Programs, Accessories, Command Prompt).

  2. Change to the folder c:\mypackages using the DOS cd command.

  3. Create a Jar file with the following line:
     jar cf stackqueue.jar stackqueue\*.class

  4. Move the file Stackqueue.jar to the folder c:\jdk1.3\jre\lib\ext.
     This assumes that jdk1.3 is the folder in which the Sun Java software has been installed.

  5. Change to the folder c:\testfolder.

  6. Compile and run TestStackQueue with these lines:
     javac TestStackQueue.java
     java TestStackQueue
     Note that no classpath directive or environment variable is needed to access Jar files placed in c:\jdk1.3\jre\lib\ext.