===== Objects =====
===== Classes & Java Files =====

All java source code text is held in clear text java files, which have a  //.java// extension.  Compiled into byte form they have an extension of //.class//.

The structure of a java file tends to be :

package <package name>

import <import file names>

<class definition>

===== Packages =====

Packages are a convenient way to group java projects together in to a //namespace//.  All the class files for an application are typically bundled into a jar file using the package structure, allowing for faster distribution of the java application.

A package may used to create a user namespace or enable access to 3rd party classes. When a particular set of classes is needed the package is listed and then the import statement calls the particular package as needed.

Practically this is represented by a file system structure that mirrors the namespace breakdown. For example a package / namespace of **org.apache.harmony** leads to a file structure of 

/org/apache/harmony

A source code example would be:

''package java.awt.event;  //allows access to java.awt.event//''

''import java.awt.event.*; //imports all the classes from java.awt.event//''

''import java.awt.event.ActionEvent; //imports the classes from java.awt.event.ActionEvent //''

A full explanation of packages is available [[http://en.wikipedia.org/wiki/Java_package|here]]


===== Common Extensions =====

There are a number of commonly used extensions in the java world.  Some of the more common ones are shown below:

^Extension^Content^Description^
|.java|Source Code|Holds the plain text of the java code|
|.class|Byte Code|Holds the compiled code derived from the plain text|
|.jar|Java Archive|Holds all the class files for a java application. A JAR file is used for aggregating many files into one. It is generally used to distribute Java classes and associated metadata.|
|.war|Web Archive|Holds the complete web application class files in one archive.a JAR file used to distribute a collection of JavaServer Pages, servlets, Java classes, XML files, tag libraries and static Web pages (HTML and related files) that together constitute a Web application.|
|.ear|Enterprise Archive|Holds a complete application for deployment to a J2EE server. An EAR file is a standard JAR file with an .ear extension, with one or more entries representing the modules of the application, and a metadata directory called META-INF which contains one or more deployment descriptors| 

Java source code is held in files with the extension .java as a plain text file.  When it is compiled into byte code it acquires the extension