Packages

Packages are containers for classes. It is just like header files in C++ and is stored in a hierarchical manner. To use a package in a class, we need to import it. There are two types of packages. They are

• System package
• User defined package.

Uses:

Packages reduce the complexity of the software, because a large number of classes can be grouped into limited number of packages.

Using packages we can hide classes.

We can create classes with same name in different packages.

System Packages:

System packages are packages which are available in the java language. They are

 java.lang, java.util, java.io, java.awt, java.net, java.applet.

java.lang:

This package contains the classes for primitive data types, strings, math functions, threads, and exceptions. If we create a class, this package will be automatically imported.

java.util:

This package contains the classes related to vectors, hash tables, random numbers, date etc.

java.io:

This package contains input/output related classes.

java.awt:

This package contains classes related to GUI. The classes contain windows, buttons, lists, menus etc.

Java.net:

This package contains classes for networking.

Java.applet:

This package contains classes related to applet.

Using system packages:

General forms:

Import  pack1[.pack2][.pack3][…]].classname;

Where,

Import – keyword

Pack1 – top level package.

Pack2 – second level package

Pack 3 – third level package

Classname – a valid class name in the package.

Eg:

import  java.awt.*;

import java.lang.math.*;

Naming conventions:     

The rules to be followed while naming a package are:

Every package name should be unique.

The first letter of the name should be a lowercase alphabet. This is to distinguish between classes and packages.

Syntax: java.name

Where java – keyword

Name – name of the package.

Eg: java.awt;

Accessing a package:

Each package contains number of related classes. We can access these classes in our application. There are two methods to access.

• By specifying the fully qualified class name.
• By using import statement.

a)By specifying the fully qualified class name:

We can access a package by specifying the class path at the time of declaration of our object. General form:

Pack1. pack2. pack n. classname obj;

b)By using import statement:

By using import statement we can either use all classes of a package in our application or a specific class. General form:

Import pack1[.pack2[.pack3[…]]].*;