Java package

JAVA Package

Prepared by

Miss. Arati A. Gadgil

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Package

Packages are containers for classes that are used to keep the class name space compartmentalized.

Java provides a mechanism for partitioning the class name space into manageable chunks.This mechanism called package.

Packages are stored in a hierarchical manner.

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Creating Package

Include a package command as the first statement in a java source file.

Any classes declared within that file will belong to the specified package.

The package statement defines a name space in which classes are stored.

If you omit the package statement , the class names are put into the default package, which has no name.

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To add a class to a package, first statement must be,

package PackageName;

keyword package tells Java that you want to add this class to a package. The name of this package will be PackageName.

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//save by A.java  package pack;  public class A{   public void msg()

{System.out.println("Hello“);

}  }

Creating package of name pack, class A is included in package pack. 

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//save by B.java  import pack.*;  class B{   public static void main(String args[])

{   A obj = new A();      obj.msg();  

  }  }

Here importing a package pack to access class A.

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Managing Source and Class Files

Many implementations of the Java platform rely on hierarchical file systems to manage source and class files.

You put the source code for a class, interface, enum or annotation in a text file whose name is the simple name of the type and whose extension is .java. Then you put the source file in a directory whose name reflects the name of the package to which the type belongs.

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For example, the source code for the SY class would be in a file named SY.java, and the file would be in a directory named marks.

The marks directory might be anywhere on the file system.

//save by SY.java  package marks;  public class SY{   }//path marks\SY.java

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If company uses its reversed Internet domain name in its package names, for example consider domain name is ResultSheet.com would precede all its package names with com.ResultSheet.

Each component of the package name corresponds to a subdirectory. So if ResuleSheet had a marks package that contained a SY.java source file, it would be contained in a series of subdirectories.

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 package com.ResultSheet.marks;  public class SY{   }//path marks\SY.java

Like  .java file,  .class file should also be in a series of directories that reflect the package name. However, it does not have to be in the same directory as its source. You could arrange your source and class directories separately.

Each directory listed in the class path is a top-level directory in which package directories appear. From the top-level directory, the compiler and the JVM can construct the rest of the path, based on the package and the class name for the class.

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In software, JAR (Java Archive) is a package file format typically used to aggregate many Java class files and associated metadata and resources (text, images, etc.) into one file to distribute application software or libraries on the Java platform.

The package java.util.zip contains classes that read and write JAR files.

A JAR file has an optional manifest file. The entries in the manifest file determine how one can use the JAR file.

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To extract the contents of a JAR file users can use any standard unzip software, or the jar command: jar –xf name.jar.

An executable Java program can be packaged in a JAR file, along with any libraries the program uses.

Executable JAR files have the manifest specifying the entry point class with Main-Class: myPrograms.MyClass and an explicit Class-Path (and the -cp argument is ignored).

Some operating systems can run these directly when clicked. The typical invocation is "java -jar name.jar" from a command line.

Creating a jar File in Command Prompt

•Start Command Prompt.

•Navigate to the folder that holds your class files: D:\jar

•Create a manifest file and your jar file:D:\jar > echo Main-Class: student >manifest.txtD:\jar > jar cvfm student.jar manifest.txt *.class

•Test your jar:D:\jar > java -jar student.jar

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Thank You

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