Describing the Java Packages

Vikas Mishra
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Package in Java

Introduction

A package is a grouping of related types providing access protection and name space management. Here type means classes, interfaces, enumerations, and annotation . Enumerations and annotation types are special kinds of classes and interfaces, respectively. So types are often referred to in this lesson simply as classes and interfaces. A Java package is a mechanism for organizing Java classes into namespaces. Thus Java Packages are containers for classes. That are used to keep the class name space compartmentalized. In Java package is mapped to a folder on your hard drive.

Java packages can be stored in compressed files called JAR files, allowing classes to download faster as a group rather than one at a time. Programmers also typically use packages to organize classes belonging to the same category or providing similar functionality. A package provides a unique namespace for the types it contains. Classes in the same package can access each other's package-access members.

Thus we can say that

Every class is part of some package.
All classes in a file are part of the same package.
You can specify the package using a package declaration:
package name ; as the first (non-comment) line in the file.
Multiple files can specify the same package name.
You can access public classes in another (named) package using:
package-name.class-name
You can access the public fields and methods of such classes using:
package-name.class-name.field-or-method-name
You can avoid having to include the package-name using:
import package-name.*; or
import package-name.class-name; at the beginning of the file (after the package declaration). The former imports all of the classes in the package, and the second imports just the named class.
You must still use:Class-name to access the classes in the packages, and
class-name.field-or-method-name to access the fields and methods of the class; the only thing you can leave off is the package name.

Some core packages of java are

java.lang	Basic language functionality and fundamental types
java.util	Collection data structure classes
java.io	File operations
java.math	Multiprecision arithmetics
java.nio	The New I/O framework for Java
java.net	Networking operations, sockets, DNS lookups
java.security	key generation, encryption and decryption
java.sql	Java Database Connectivity to access databases
java.awt	Basic hierarchy of packages for native GUI components
javax.swing	Hierarchy of packages for platform-independent rich GUI
java.applet	Classes for creating an applet

Create It's Own package

For creating a package we follow the following steps:

Defining a Package

To define a 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.If you omit the package statement, the class names are put into the default package, which has no name.This is the general form of the package statement:

package packageName;

Create a hierarchy of packages

To create a hierarchy of packages, separate each package name from the one above it by use of a period. The general form of a multileveled package statement:

package pkg1[.pkg2[.pkg3]];

Example: In this example we create a folder of name MyPack and save this java file with name My1.java .The package name and folder name should be same:

My1.java

package MyPack;

public class My1 {
public static void main(String args[])
   {
    System.out.println("Hi Friends");
    System.out.println("My name is Vikas Mishra");
    System.out.println("This application is the example of How to create it's own package in java");
    System.out.println("Hope its help you to understand the package concept");
   }
}

Compile and Run it

After running this code will give the output as follow:

Using The packages

For using the java core packages we Importing the packages and for importing we use the import statement .In a Java source file, import statements occur immediately following the package statement and before class definitions.

This is the general form of the import statement:
import pkg1[.pkg2].(classname|*);
(*) means including all classes under that package.
For example:

import java.lang.*;

An Another Application

In this example we are going to import util package to use the class:

A. java:

import java.util.Date;

public class A {
public static void main(String args[])
{
System.out.println(new Date());
System.out.println("My name is vikas");
System.out.println("This is an example of using A java package util");
}
}