Java Packages

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Java Packages

• B.Ramamurthy

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Topics for Discussion

• Keyword package
• Packaging a set of classes
• CLASSPATH reference to a package
• Example
• Creating a Java Archive (jar)
• Using a jar file
• Summary

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Keyword package

• Package construct is used to package a set of
  related class.
• You will find many examples in Java API
  java.util package, java.lang package etc.
• To include a class into a package add a
  statement
• package package_name
• before the class header.

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package Definition

• 1. Add package package_name to all the java
  source files that contains class definition to be
  included in the package.
• 2. Create a directory package_name
• 3. Compile and place the class files in a
  directory package_name

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package Usage

• 1. Update your CLASSPATH environment variable to
  include the path where the directory is located.
• 2. Make sure the update is effective by executing
  source .cshrc
• 3. Include import package_name. in the
  application/applets where you want to use these
  classes.

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Example

• Look at ch17, fig17_04
• The application is in CreateSequentialFile.java.
• The classes it uses are BankU.javaI and
  BankAccountRecord.java
• Lets create a package ch17 out of these two
  classes.
• Create a directory ch17 anywhere you want. Move
  BankUI.java and BanAccountRecord.java into this
  directory.
• Add package ch17 at the top of the two files.
  Remove any other reference to packages.

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Example (contd.)

• Compile the files BankUI.java and
  BankAccountRecord.java
• Update .cshrc to include the path to the parent
  directory of ch17.
• source .cshrc
• Update CreateSequentialFile.java to import
  classes from ch17
• import ch17.
• Compile and execute.

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Archiving the Package

• Often once the classes are related by package,
  and after they are tested well, they are
  compressed using a jar tool to provide an easy
  way to transport the package as one.
• Command (See man jar)
• jar cf jar_file_name classes to be archived
• Examples
• jar ch17.jar BankUI.class Record.class
  BankAccountRecord.class

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Including jar file in CLASSPATH

• When a package is in the form of a .jar file, you
  need actually include the jar file in the
  CLASSPATH.
• Example
• setenv CLASSPATH ./projects/bina/ch17.jar

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Jar your files for Prj2

• Create a jar files out of all the source files
  (java) into a prj2.jar
• Submit prj2.jar file.
• One more modification No need for print in prj2.
  We will do that prj3.
• But save and load are needed. These can be hidden
  operations, that is, no need for an user
  interface for data saving and restoring.

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Summary

• You can collect all the useful classes you have
  created into a package.
• Create a CS2 package.
• Use it in your future applications.