Java package

A Java package organizes Java classes into namespaces, providing a unique namespace for each type it contains. Classes in the same package can access each other's package-private and protected members. Java packages can be stored in compressed files called JAR files, allowing classes to be downloaded faster as groups rather than individually.

In general, a package can contain the following kinds of types: classes, interfaces, enumerations, and annotation 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. Programmers also typically use packages to organize classes belonging to the same category or providing similar functionality.

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. At most one package declaration can appear in a source file.

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

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

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:

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

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. Classes in an unnamed package cannot be imported by classes in any other package.

The official Java Tutorial advises against this:

Public members and classes are visible everywhere and private members are visible only in the same class. Classes within a package can access classes and members declared with default (package-private) access as well as class members declared with the protected access modifier. Default (package-private) access is enforced when a class or member has not been declared as public, protected or private. By contrast, classes in other packages cannot access classes and members declared with default access. However, 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.

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