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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.
In Java source files, the package that the file belongs to is specified with the
package keyword. This keyword is usually the first keyword in source file.
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
imports all classes from the
java.awt.event package, while
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();
does not require a preceding import statement.
Package access protection
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
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.
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 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 their 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 US company named MySoft also creates a fractions package, but names it us.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.
Core packages in Java SE 6
|Template:Javadoc:SE||— basic language functionality and fundamental types|
|Template:Javadoc:SE||— collection data structure classes|
|Template:Javadoc:SE||— file operations|
|Template:Javadoc:SE||— multiprecision arithmetics|
|Template:Javadoc:SE||— the New I/O framework for Java|
|Template:Javadoc:SE||— networking operations, sockets, DNS lookups, ...|
|Template:Javadoc:SE||— key generation, encryption and decryption|
|Template:Javadoc:SE||— Java Database Connectivity (JDBC) to access databases|
|Template:Javadoc:SE||— basic hierarchy of packages for native GUI components|
|Template:Javadoc:SE||— hierarchy of packages for platform-independent rich GUI components|
|Template:Javadoc:SE||— classes for creating and implementing applets|
- Java Language Specification, 3rd Edition, Chapter 7: Packages
- Code Conventions for the Java Programming Language, Chapter 9: Naming conventions (mentions packages)
- Sun's "Java Tutorials" Lesson: Packages