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Putting a class into a package

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• Commonly used organization structure
  • Organization structure

    The hierarchical structure is a commonly used structure for organization

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  • Examples of organizing topics into hierarchies:

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• Putting a class into a package
  • You can put a class into a package using the package clause.

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  • Example: the Hello World program inside the package myLib:

    package myLib; // package clause public class TestPackage { public static void main(String[] args) { System.out.println("Hello World"); } }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    mkdir ~/demo mkdir ~/demo/myLib Right click on link and save in the directory named ~/demo/myLib. To compile:   (cd ~/demo; javac myLib/TestPackage.java) To run:          (cd ~/demo; java myLib.TestPackage)

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  • Java programming rule:

    A class ClassA that is stored inside a package PackageX must be stored inside a (UNIX) directory named PackageX. Illustrated:

    Example:

    The Hello World Java program that is defined inside the package myLib must be stored inside a directory named myLib.

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  • Experiment:

    Save this program ( click here ) in some directory (not named myLib) Compile it with: javac TestPackage.java Run it from within the stored directory with: java TestPackage You will get this run time error: >>> java TestPackage Exception in thread "main" java.lang.NoClassDefFoundError: TestPackage (wrong name: myLib/TestPackage)

    Explanation:

    Because the class TestPackage contains the package clause package myLib;, the complete class name of the class is in fact: myLib.TestPackage. Because the official class name is myLib.TestPackage, it must be access as myLib.TestPackage. (In other words: it cannot be accessed without the package name).

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• Compiling and running methods in a class stored inside a package
  • When compiling a class or running (executing) methods of a class inside a package, your working directory should be:

    the directory that contains the directory of the package.

    Example:

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  • To compile a java program file in package packageX, use the command:

    cd Dir (cd to the directory that contains packageX) java packageX/javaProgram.java

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  • To run a java program file in package packageX, use the command:

    cd Dir (cd to the directory that contains packageX) java packageX.javaProgram