Using packages
In a Java source file, the package that this file's class or classes belong to is specified with thepackage
keyword. This keyword is usually the first keyword in the source file. At most one package declaration can appear in a source file.
import
declaration. The following declaration
java.awt.event
package, while the next declaration
ActionEvent
class from the package. After either of these import declarations, the ActionEvent
class can be referenced using its simple class name:
Package-wide Javadoc & annotations
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.The unnamed 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. The official Java Tutorial advises against this: :Generally speaking, an unnamed package is only for small or temporary applications or when you are just beginning the development process. Otherwise, classes and interfaces belong in named packages.Package access protection
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.
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.
, 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 almost 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 its package. Subsequent components of the package name vary according to an organization's own internal naming conventions.
For example, if an organization in Canada called MySoft creates a package to deal with fractions, naming the package distinguishes the fractions package from another similar package created by another company. If a German company named MySoft also creates a fractions package, but names it , 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 8
Modules
InReferences
External links
* {{Javadoc:SE Package