package in java

package in java:->
A Java package is a mechanism for organizing Java classes into namespaces similar to the
modules of Modula. 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.


In general, a package can contain the following kinds of types. A package allows
a developer to group classes (and interfaces) together. These classes will all be
related in some way – they might all have to do with a specific application or perform
a specific set of tasks. The Java API is a collection of packages, for example the javax.xml
package. The javax.xml package and its subpackages contain classes to handle XML.


In a Java source file, the package that this file's class or classes belong to is specified
with the package keyword. This keyword is usually the first keyword in the source file.[1]
package java.awt.event;
To use a package's classes inside a Java source file, it is convenient to import the classes
 from the package with an import declaration. The following declaration
import java.awt.event.*;
imports all classes from the java.awt.event package, while the next declaration
import java.awt.event.ActionEvent;
imports only the ActionEvent class from the package. After either of these import
 declarations, the ActionEvent class can be referenced using its simple class name:
ActionEvent myEvent = new ActionEvent();
Classes can also be used directly without an import declaration by using the fully
 qualified name of the class. For example,
java.awt.event.ActionEvent myEvent = new java.awt.event.ActionEvent();
does not require a preceding import declaration.
Note that if you do not use a package declaration, your class ends up in an unnamed
package.[2][3] Classes in an unnamed package cannot be imported from classes in any other package.[4]


Classes within a package can access classes and members declared with default
access and class members declared with the protected access modifier. Default
 access is enforced when neither the public, protected nor private access modifier
is specified in the declaration. By contrast, classes in other packages cannot access
classes and members declared with default access. Class members declared as protected
can be accessed from the classes in the same package as well as classes in other packages
that are subclasses of the declaring class.
Creation of JAR files

JAR Files are created with the jar command-line utility. The command
jar cf myPackage.jar *.class
compresses all .class files into the JAR file myPackage.jar. The ' c ' option on the command
line tells the jar command to "create new archive." The ' f ' option tells it to create a file.
 The file's name comes next before the contents of the JAR file.
[edit]Package naming conventions

Packages are usually defined using a hierarchical naming pattern, with levels in the hierarchy
 separated by periods (.) (pronounced "dot"). Although packages lower in the naming hierarchy
are often referred to as "subpackages" of the corresponding packages higher in the hierarchy,
there is almost no semantic relationship between packages. The Java Language Specification establishes
 package naming conventions to avoid the possibility of two published packages having the same name.
The naming conventions describe how to create unique package names, so that packages that are widely
distributed will have unique namespaces. This allows packages to be separately, easily and automatically
installed and catalogued.
In general, a package name begins with the top level domain name of the organization and then the
organization's domain and then any subdomains, listed in reverse order. The organization can then
choose a specific name for its package. Package names should be all lowercase characters whenever possible.
For example, if an organization in Canada called MySoft creates a package to deal with fractions,
 naming the package ca.mysoft.fractions distinguishes the fractions package from another similar
package created by another company. If a German company named MySoft also creates a fractions package,
but names it de.mysoft.fractions, then the classes in these two packages are defined in a unique and
 separate namespace.
Complete conventions for disambiguating package names and rules for naming packages when the Internet
 domain name cannot be directly used as a package name are described in section 7.7 of the Java Language Specification.