Java in a Nutshell, 5th Edition

Packages and the Java Namespace

A package is a named collection of classes, interfaces, and other reference types. Packages serve to group related classes and define a namespace for the classes they contain.

The core classes of the Java platform are in packages whose names begin with java. For example, the most fundamental classes of the language are in the package java.lang. Various utility classes are in java.util. Classes for input and output are in java.io, and classes for networking are in java.net. Some of these packages contain subpackages, such as java.lang.reflect and java.util.regex. Extensions to the Java platform that have been standardized by Sun typically have package names that begin with javax. Some of these extensions, such as javax.swing and its myriad subpackages, were later adopted into the core platform itself. Finally, the Java platform also includes several "endorsed standards,” which have packages named after the standards body that created them, such as org.w3c and org.omg.

Every class has both a simple name, which is the name given to it in its definition, and a fully qualified name, which includes the name of the package of which it is a part. The String class, for example, is part of the java.lang package, so its fully qualified name is java.lang.String.

This section explains how to place your own classes and interfaces into a package and how to choose a package name that won’t conflict with anyone else’s package name. Next, it explains how to selectively ...