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Concept
Java package
Summary
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.
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.
package java.awt.event;
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
import java.awt.event.*;
imports all classes from the java.awt.event package, while the next declaration
import java.awt.event.ActionEvent;
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:
ActionEvent myEvent = new ActionEvent();
Classes can also be used directly without an import declaration 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 declaration.
Documentation explaining the package as a whole is written as Javadoc in a file named exactly
package-info.java. That file is also the place for annotations to be used across all classes of the package.
If a package declaration is not used, classes are placed in an unnamed package. Classes in an unnamed package cannot be imported by classes in any other package.Official source
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