JAVA PACKAGES

    Packages are Java’s way of grouping a variety of classes and/or interfaces together. The grouping is usually done according to functionality.

   Packages act as “Container” for classes.

     Packages provides the following benefits:

o   The classes contained in the packages of other programs can be easily reused.

o   In packages, classes can be unique compared with other classes in other packages. That is, two classes in two different packages can have same name. They may be referred by their fully qualified name, comprising the package name and the class name.

o   Packages provide a way to hide classes and thus preventing other program or packages from accessing classes that are meant for internal use only.

o   Packages also provide a way for separating “design” from “coding”. First we can design classes and decide their relationships, and then we can implement the Java code needed for the methods.

    Java packages are classified into two catagories: Java API packages,

o   Java API Packages: Java API provides a large number of classes grouped into different packages according to functionality. The following table shows the classes that belong to each package:

Package Name	Contents
java.lang	Language support classes. These are classes that Java compiler itself uses and therefore they are autometically imported. They include classes for primitive types, strings, math functions, threads and exceptions.
java.util	Language utility classes such as vectors, hash tables, random numbers, date, etc.
java.io	Imput/Output support classes. They provide facilities for the input and output of data.
java.awt	Set of classes for implementing graphical user interface. They include classes for windows, button, list, menus and so on.
java.net	Classes for networking. They include classes for communicating with local computers as well as with internet server.
java.applet	Classes for creating and implementing applets.

o   Using System Packages: There are two ways to access the classes stored in a package:

First, Use the fully qualified class name of the class that we want to use. Like, to refer to class Color in the awt package, the following command is used:

java.awt.Colour

This approach is useful if one need to access the class only once or when it is not required to access any other classes of the package.

Second method is to use the import statementand must appear at the top of the file, before any class declaration. It should be used in the following way:

                             import packagename.classname;

                                                or

                             import packagename.*;

Ex: import java.awt.Color;

       import java.awt.*; //bring all classes of java.awt package

o   Naming Conventions: double y=java.lang.Math.sqrt(x);

                                                        package   class   method

                                                name     name    name

o   Creating Packages:The steps that are followed for creating our own packages are as follows:

§  Declare the package at the beginning of a file using the form:

package packagename;

§  Define the class that is to be put in the package and declare it public.

§  Create a subdirectory under the directory where the main source files are stored.

§  Store the listing as the classname.java file in the subdirectory created.

§  Compile the file. This creates .class file in the subdirectory.

§  The name of the subdirectory must match the package name exactly.

§  Java also supports the concept of package hierarchy. This is done by specifying multiple names in a package statement, separated by dots.

package firstpackage.secondpackage;

§  A Java package file can have more than one class definition. In such a case, only one of the classes may be declared public and that class name with .java extension is the source file name. When a source file with more than one class definition is compiled, Java creates independent .class files for those classes.

o   Using a Package:

Ex:Define a package:

package Mypack;

public class Display

{

 public void show()

 {

   System.out.println("\nShow method in package Mypack in display is called");

 }

}

class Design

{

 void show()

 {

   System.out.println("\nShow method in package Mypack in design is called");

 }

}

Define a class:

import Mypack.*;

class Package{

 public static void main(String args[])

 {

  Display d=new Display();

  d.show();

//Design d1=new Design();

//d1.show();

// Design is not public hence can’t be accessed from outside of package.

 }

}