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Packaging in Java
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Packaging
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  • The Concept:
    • Group/organize components
  • The Rationale:
    • "Chunking"
    • Provides a higher-level view (abstraction)
    • Increases cohesion
    • Reduces coupling
    • Has an organizational role similar to directories/folders
Identifying/Creating Packages
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  • Components with a common purpose:
    • The package indicates the types are related
  • Components that participate in the same use cases:
    • The package helps identify the components that provide the necessary functionality
  • Components that are strongly associated:
    • The package allows access across components
Namespaces in Java
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  • Packages Define a Namespace:
    • Two packages can contain components with the same name
    • Package names can be prepended to class names (e.g., plant.Leaf and hierarchy.Leaf) to create a unique, fully-qualified name
  • The Namespace Appears Hierarchical but Isn't:
    • A package can appear to contain another package (e.g., java.awt appears to contain java.awt.event) but it doesn't
Namespaces in Java (cont.)
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  • Ensuring Uniqueness:
    • Use a package name that uniquely identifies the programmer or organization
  • A Common Convention:
    • Use a a "flipped" domain (e.g., edu.jmu.cs.package )
  • A Detail:
    • Java naming conventions still apply so inappropriate elements should be preceded by an underscore (e.g., com._static.package )
Some Java Packages
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javapackages
Using Classes in Packages
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  • Classes in java.lang:
    • Nothing needs to be done to use classes in java.lang (e.g., the String class)
  • Classes in Other Packages:
    • Use the fully-qualified name (e.g., java.util.ArrayList)
Using Classes in Packages (cont.)
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  • An Observation:
    • Using the fully-qualified name can be very inconvenient (and verbose)
  • A Remedy:
    • Use import statements to tell the compiler/interpreter how to try and resolve unidentified symbols and then use the unqualified name of the class
    • A Specific Class: import package.class;
    • All Classes: import package.*;
  • Disambiguating:
    • Sometimes you must use the fully-qualified name (e.g., to use both java.awt.List and java.util.List in the same class
Packages and the File System
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  • The Normal Situation:
    • Each package corresponds to a directory and the apparent hierarchy is realized as an actual hierarchy (e.g., the package statistics.parametric is in the directory parametric inside of the directory statistics
  • An Important Point:
    • Packages map to a part of the file system (i.e., a package is defined relative to some top-level directory)
  • Specifying The Top Level:
    • When compiling/executing from the command shell: Done using the environment space (i.e., CLASSPATH) or with a compiler/interpreter switch (i.e., -cp or -classpath)
    • When compiling/executing from an IDE: Done by adding the packages to the project
Creating Packages
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  • Create the Directory Structure:
    • The directory structure (under the top-level directory) should match the package structure
  • Include the package Statement in each Class:
    • package package;
A Convenience
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  • Using static Members:
    • The fully-qualified name is package.Class.member
  • Using static Members from Imported Packages:
    • You can omit the package name and refer to them as Class.member
  • Static Imports:
    • If you use import static you can access static members without including the package name or the class name (e.g., abs() rather than Math.abs())
Visibility Revisited
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  • Visibilities You Are Familiar With:
    Block Entity is accessible within a block of code
    Subprogram Entity is accessible within a subprogram
    Class/Private Entity is accessible by all entities within a class
    Generalized/Protected Entity is accessible in its class and all extensions
    Complete/Public Entity is accessible everywhere
  • Another Kind of Visibility:
    Group Entity is accessible to all members of a pre-defined group (e.g., package visibility in Java, COMMON blocks in FORTRAN)
Visibility Revisited (cont.)
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  • Package Visibility in Java:
    • Attributes are visible to all classes in the same package
    • Implemented by not specifying an explicit visibility
  • Protected Visibility in Java Revisited:
    • Attributes are actually visible to subclasses and other classes in the same package
  • Package Visibility in UML
    • Denoted using a ~
Deployment
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  • An Observation:
    • Directory trees are a good way to organize packages
    • It is often inconvenient to move directory trees around
  • One Solution:
    • Java Archives (.jar Files)
Java Archives
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  • Simple Archives:
    • Multiple classes/packages combined in a single file
    • Created using the jar tool
    • Can be included in the CLASSPATH
  • Executable Archives:
    • Can be made to appear "executable" (though they are actually interpreted by java or javaw)
Java Archives (cont.)
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  • Resources:
    • One sometimes wants to include files other than .class files in a .jar file (e.g., images, configuration files, input files)
    • Such files are often called resources
  • "Loading" Resources:
    • File objects normally refer to files in the file system
    • To refer to a resource in a .jar file you should use a URI object
    • Constructing the URI is a multi-step process that uses the getResource() method in the Class class
Creating Java Archives
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  • From the Command Line:
    • Compile all of the classes
    • Create a manifest file (e.g., named manifest.mf): 

                  Manifest-Version: 1.2
            Main-Class: Driver

      Note: The class containing the main(String[]) method needn't be named Driver.

    • Create the jar file (e.g., named application.jar): 

                  jar cvfm application.jar manifest.mf *.class

  • From jGRASP:
    • Create a project
    • Use "Project"-"Create JAR..."
  • From Eclipse:
    • Create a project
    • Use "File"-"Export"-"Java"-"Jar File"
There's Always More to Learn
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