Java package

Java source files belonging to the same category or provide similar functionality can be packed into a Java package. Java packages are stored in compressed files called JAR files. Inside the package the files can be stored hierachically, therefore it is possible to combine several subdirectories into one package.

• A package provides a unique namespace for the types it contains.
• A package facilitates access protection.
• A package can contain the following kinds of types.
  • Classes
  • Interfaces
  • Enumerated Types
  • Annotations

Using packages

To use a package inside a Java source file, it is convenient to import the classes from the package with an import statement. The statement

import java.awt.event.*;

imports all classes from the java.awt.event package, while

import java.awt.event.ActionEvent;

imports only the ActionEvent class from the package. After either of these import statements, the ActionEvent class can be referenced using its simple class name:

ActionEvent myEvent = new ActionEvent();

Classes can also be used directly without an import statement by using the fully-qualified name of the class. For example,

java.awt.event.ActionEvent myEvent = new java.awt.event.ActionEvent();

doesn't require a preceding import statement.

Creation of JAR Files

In Java source files the package the file belongs to is specified with the package keyword.

package java.awt.event;

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.

Package Naming Conventions

In general, a package name begins with the top level ___domain name of the organization and then the organization's ___domain. The organization can then choose a specific name for their pacakge. 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, they might name their package: ca.mysoft.fractions

Packages in J2SE 1.5

• java.lang, java.util: These package hierarchies provide basic language functionality like types, regular expressions, the Reflection API
• java.math: provides mathematical functions
• java.io: file operations
• 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 (JDBC) to access databases
• java.awt: basic hierarchy of packages for native GUI components
• javax.swing: hierarchy of packages for platform-independent rich GUI components

External links

• Java SE 24 API Javadocs
• Java Package Naming Conventions [1]