Java Package | Java API Package | Importing Package In Java

                               Java Package

A java package is a group of similar types of classes, interfaces and sub-packages.

Package in java can be categorized in two form, built-in package and user-defined package.

There are many built-in packages such as java, lang, awt, javax, swing, net, io, util, sql etc.

Package in Java is a mechanism to encapsulate a group of classes, sub packages and interfaces.

  Packages are used for:

·         Preventing naming conflicts. For example there can be two classes with name Employee in two packages, college.staff.cse.Employee and college.staff.ee.Employee

·    Making searching/locating and usage of classes, interfaces, enumerations and annotations easier

·         Providing controlled access: protected and default have package level access control. A protected member is accessible by classes in the same package and its subclasses. A default member (without any access specifier) is accessible by classes in the same package only.

·         Packages can be considered as data encapsulation (or data-hiding).

Fig: Java Package

Advantage of Java Package

1) Java package is used to categorize the classes and interfaces so that they can be easily maintained.

2) Java package provides access protection.

3) Java package removes naming collision.

Simple example of java package

The package keyword is used to create a package in java.

    //save as Simple.java  

    package mypack;  

    public class Simple{  

    public static void main(String args[]){  

    System.out.println("Welcome to package");  

   }  

    }  

How to compile java package

syntax –

javac -d directory javafilename  

 

Example-

javac -d . Simple.java  

How to run java package program

You need to use fully qualified name e.g. mypack.Simple etc to run the class.

To Compile: javac -d . Simple.java

To Run: java mypack.Simple     Output: Welcome to package Adding a class to a Package :  We can add more classes to a created package by using package name at the  top of the program and saving it in the package directory. We need a new  java file to define a public class, otherwise we can add the new class to  an existing .java file and recompile it. Subpackages:  Packages that are inside another package are the subpackages.  These are not imported by default, they have to imported explicitly. Also,  members of a subpackage have no access privileges, i.e., they are considered  as different package for protected and default access specifiers. Example : importjava.util.*;

Program:-

util is a subpackage created inside java package.

// Java program to demonstrate accessing of members when

// corresponding classes are imported and not imported.

import java.util.Vector;

public class ImportDemo

{

public ImportDemo()

{

          // java.util.Vector is imported, hence we are

          // able to access directly in our code.

          Vector newVector = new Vector();

          // java.util.ArrayList is not imported, hence

          // we were referring to it using the complete

          // package.

          java.util.ArrayList newList = new java.util.ArrayList();

}

public static void main(String arg[])

{

          new ImportDemo();

}

}

Fig: Predefined package