Java package

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The Java package sets the Java class to the namespace, providing a unique namespace for each type it contains. The classes in the same package can access each of the privately-protected packages and members. Java packages can be stored in compressed files called JAR files, allowing the class to be downloaded faster as a group rather than individually.

In general, a package can contain the following types of types: classes, interfaces, enumerations, and annotation types. Packages allow developers to group classes (and interfaces) simultaneously. These classes will all be related in some way - they may all have to be done with a particular application or perform a specific set of tasks. Programmers also typically use packages to organize classes that belong to the same category or provide similar functionality.

Video Java package

Use package

In the Java source file, the package that the class file or class belongs to is specified with the package keyword. This keyword is usually the first keyword in the source file. At most one package declaration may appear in the source file.

To use the package class inside the Java source file, it is easier to import the class from the package with the import declaration. The following declaration

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

just import the ActionEvent class from the package. After one of these import declarations, the ActionEvent class can be referenced using a simple class name:

Classes can also be used directly without import declarations by using fully qualified class names. As an example,

does not require prior import declaration.

The nameless package

Note that if you do not use package declarations, your class will end up in an unnamed package. Classes in unnamed packages can not be imported by classes in other packages.

The official Java tutorial suggests against this:

Generally, unnamed packages are only for small or temporary applications or when you are just starting the development process. Otherwise, classes and interfaces are included in named packets.

Maps Java package

Packet access protection

Members and public classes can be seen everywhere and private members can only be seen in the same class. The classes in the package can access the class and the declared member with the default ( private-packet access) as well as the class members declared with protected access modifier. The default access (private-package) is applied when the class or members have not been declared as public , protected or private . Conversely, classes in other packets can not access the class and members declared with default access. However, class members expressed as protected can be accessed from classes in the same package as well as classes in other packages that are subclasses of the class that it claims.

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JAR file creation

The JAR file is created with the jar command line utility. Command

 jar cf myPackage.jar *.class  

compress all.class files into the JAR file myPackage.jar . The 'c' option on the command line tells the jar command to "create a new archive." The 'f' option tells it to create the file. The filename appears before the contents of the JAR file.

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Package naming convention

Packages are usually defined using hierarchical naming patterns, with multiple levels in the hierarchy separated by periods (. , pronounced "dot"). Although the lower packets in the naming hierarchy are often referred to as "sub-packages" of the corresponding packets higher in the hierarchy, there is hardly any semantic relation between the packets. The Java Language Specification assigns a package naming convention to avoid the possibility of two published packages having the same name. The naming convention illustrates how to create a unique package name, so the widely distributed package will have a unique namespace. It allows packages to be separated, easily and automatically installed and cataloged.

In general, the package name starts with the top level domain name of the organization and then the domain of the organization and then each subdomain, listed in reverse order. The organization can then choose a specific name for the package. The next component of the package name varies according to the organization's own internal naming conventions.

For example, if an organization in Canada named MySoft creates a package to handle fractions, giving the package name ca.mysoft.fractions distinguishes fractional packages from other similar packages created by other companies. If a German company named MySoft also creates a fraction pack, but give it the name de.mysoft.fractions , then the classes in these two packages are specified in a separate and unique namespace.

Complete provisions for confusing package names and rules for naming packages when Internet domain names can not be used directly as packet names are described in section 7.7 Java Language Specifications.

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The core package in Java SE 8

The java.lang package is available without using import statements.

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Module

In Java 9, "modules", a package package, are planned as part of Project Jigsaw; this was previously called "superpackages" and originally planned for Java 7.

The module will explain their dependencies in the module declaration to be placed in a file named module-info.java at the source-file hierarchy of the module. JDK will be able to check both at compile and processing time. The JDK itself will be modulated for Java 9.

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References

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External links

• Java SE 10 API Javadocs

Source of the article : Wikipedia