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<title>Java package</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Java package</span></span>

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<p>A <b>Java package</b> organizes <a href="Java_(programming_language)" title="Java (programming language)">Java</a> <a href="Class_(computer_science)" class="mw-redirect" title="Class (computer science)">classes</a> into <a href="Namespaces" class="mw-redirect" title="Namespaces">namespaces</a>,<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

providing a unique namespace for each type it contains.

Classes in the same package can access each other's package-private and protected members.

</p><p>In general, a package can contain the following kinds of <a href="Datatypes" class="mw-redirect" title="Datatypes">types</a>: classes, <a href="Interface_(Java)" title="Interface (Java)">interfaces</a>, enumerations, records and <a href="Java_annotation" title="Java annotation">annotation</a> 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.

</p>

<div class="mw-heading mw-heading2"><h2 id="Using_packages">Using packages</h2></div>

<p>In a Java source file, the package that this file's class or classes belong to is specified with the

<code>package</code> <a href="Keyword_(computer_programming)" class="mw-redirect" title="Keyword (computer programming)">keyword</a>. This keyword is usually the first keyword in the source file. At most one package declaration can appear in a source file.

</p>

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre><span class="kn">package</span><span class="w"> </span><span class="nn">java.awt.event</span><span class="p">;</span>

</pre></div>

<p>To use a package's classes inside a Java source file, it is convenient to import the classes from the package with an <code>import</code> declaration. The following declaration

</p>

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre><span class="kn">import</span><span class="w"> </span><span class="nn">java.awt.event.*</span><span class="p">;</span>

</pre></div>

<p>imports all classes from the <code>java.awt.event</code> package, while the next declaration

</p>

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre><span class="kn">import</span><span class="w"> </span><span class="nn">java.awt.event.ActionEvent</span><span class="p">;</span>

</pre></div>

<p>imports only the <code>ActionEvent</code> class from the package. After either of these import declarations, the <code>ActionEvent</code> class can be referenced using its simple class name:

</p>

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre><span class="n">ActionEvent</span><span class="w"> </span><span class="n">myEvent</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">ActionEvent</span><span class="p">();</span>

</pre></div>

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

</p>

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre><span class="n">java</span><span class="p">.</span><span class="na">awt</span><span class="p">.</span><span class="na">event</span><span class="p">.</span><span class="na">ActionEvent</span><span class="w"> </span><span class="n">myEvent</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">new</span><span class="w"> </span><span class="n">java</span><span class="p">.</span><span class="na">awt</span><span class="p">.</span><span class="na">event</span><span class="p">.</span><span class="na">ActionEvent</span><span class="p">();</span>

</pre></div>

<p>does not require a preceding import declaration.

</p>

<div class="mw-heading mw-heading3"><h3 id="Package-wide_Javadoc_&_annotations">Package-wide Javadoc & annotations</h3></div>

<p>Documentation explaining the package as a whole is written as <a href="Javadoc" title="Javadoc">Javadoc</a> 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.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

</p>

<div class="mw-heading mw-heading3"><h3 id="The_unnamed_package">The unnamed package</h3></div>

<p>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.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> The official Java Tutorial advises against this:

</p>

<dl><dd>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.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup></dd></dl>

<div class="mw-heading mw-heading2"><h2 id="Package_access_protection">Package access protection</h2></div>

<p>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 <i>default</i> (<i>package-private</i>) access as well as class members declared with the <i><code>protected</code></i> access modifier. Default (package-private) access is enforced when a class or member has not been declared as <code>public</code>, <code>protected</code> or <code>private</code>. By contrast, classes in other packages cannot access classes and members declared with default access. However, class members declared as <code>protected</code> can be accessed from the classes in the same package as well as classes in other packages that are subclasses of the declaring class.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

</p>

<div class="mw-heading mw-heading2"><h2 id="Creation_of_JAR_files">Creation of JAR files</h2></div>

<p>JAR files are created with the jar command-line utility. The command

</p>

<pre>jar cf myPackage.jar *.class

</pre>

<p>compresses all .class files into the JAR file <i>myPackage.jar</i>. 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.

</p>

<div class="mw-heading mw-heading2"><h2 id="Package_naming_conventions">Package naming conventions</h2></div>

<p>Packages are usually defined using a <a href="Hierarchical" class="mw-redirect" title="Hierarchical">hierarchical</a> naming <a href="Pattern" title="Pattern">pattern</a>, with some levels in the hierarchy separated by periods (<code>.</code>, 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.

</p><p>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.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup>

</p><p>For example, if an organization in Canada called MySoft creates a package to deal with fractions, naming the package <code>ca.mysoft.fractions</code> 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 <code>de.mysoft.fractions</code>, then the classes in these two packages are defined in a unique and separate namespace. There is no requirement that the TLD must be the name of the country of the company. For instance, many packages also use other TLDs such as <code>org</code>, <code>com</code>, etc. The Java standard library puts all symbols in the <code>java</code> or <code>javax</code> namespace, though some classes for XML parsing reside in <code>org.w3c</code>. Some projects, such as Project Lombok, do not even use TLDs at all (all symbols are located in namespace <code>lombok</code>).

</p><p>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.<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Core_packages_in_Java_SE_8">Core packages in Java SE 8</h2></div>

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</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Java_Platform%2C_Standard_Edition" title="Java Platform, Standard Edition">Java Platform, Standard Edition</a></div>

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<td>Basic language functionality and fundamental types. Implicitly imported by every program.

</td></tr>

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<td>Collection <a href="Data_structure" title="Data structure">data structure</a> classes

</td></tr>

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<td>File operations

</td></tr>

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<td>Multiprecision arithmetics

</td></tr>

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<td>The <a href="Non-blocking_I/O_(Java)" title="Non-blocking I/O (Java)">Non-blocking I/O</a> framework for Java

</td></tr>

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<td>Networking operations, sockets, <a href="DNS_lookup" class="mw-redirect" title="DNS lookup">DNS lookups</a>, ...

</td></tr>

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<th scope="row"><code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">java.security</code>

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<td>Key generation, encryption and decryption

</td></tr>

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<td><a href="Java_Database_Connectivity" title="Java Database Connectivity">Java Database Connectivity</a> (JDBC) to access databases

</td></tr>

<tr>

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<td>Basic hierarchy of packages for native GUI components

</td></tr>

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<td>Provides classes and interfaces for handling text, dates, numbers, and messages in a manner independent of natural languages.

</td></tr>

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</th>

<td>Provides the RMI package.

</td></tr>

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<td>The main API for dates, times, instants, and durations.

</td></tr>

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<th scope="row"><code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">java.beans</code>

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<td>The java.beans package contains classes and interfaces related to JavaBeans components.

</td></tr>

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<th scope="row"><code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">java.applet</code>

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<td>This package provides classes and methods to create and communicate with the applets.

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<div class="mw-heading mw-heading2"><h2 id="Modules">Modules</h2></div>

<div role="note" class="hatnote navigation-not-searchable">Further information: <a href="Java_Platform_Module_System" title="Java Platform Module System">Java Platform Module System</a></div>

<p>In <a href="Java_9" class="mw-redirect" title="Java 9">Java 9</a> (released on September 21, 2017) support for "<a href="Modular_programming" title="Modular programming">modules</a>", a kind of collection of packages, was implemented as a result of the development effort of Project Jigsaw. The "modules" were earlier called "superpackages" and originally planned for Java 7.

</p><p>Modules describe their dependencies in a declaration placed in a file named <i>module-info.java</i> at the root of the module's source-file hierarchy. Since <a href="Java_version_history#Java_SE_9" title="Java version history">Java 9</a>, the JDK is able to check the module dependencies both at compile time and runtime. The JDK itself is modularized for <a href="Java_version_history#Java_SE_9" title="Java version history">Java 9</a>.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> For example, the majority of the Java standard library is exported by the module <code>java.base</code>.

</p><p>As an example, the following module declaration declares that the module <i>com.foo.bar</i> depends on another <i>com.foo.baz</i> module, and exports the following packages: <i>com.foo.bar.alpha</i> and <i>com.foo.bar.beta</i>:

</p>

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre><span class="k">module</span><span class="w"> </span><span class="n">com</span><span class="p">.</span><span class="n">foo</span><span class="p">.</span><span class="n">bar</span><span class="w"> </span><span class="p">{</span>

<span class="w"> </span><span class="k">requires</span><span class="w"> </span><span class="n">com</span><span class="p">.</span><span class="n">foo</span><span class="p">.</span><span class="n">baz</span><span class="p">;</span>

<span class="w"> </span><span class="n">exports</span><span class="w"> </span><span class="n">com</span><span class="p">.</span><span class="n">foo</span><span class="p">.</span><span class="n">bar</span><span class="p">.</span><span class="n">alpha</span><span class="p">;</span>

</pre></div>

<ul><li><a href="Translation_unit_(programming)" title="Translation unit (programming)">Translation unit (programming)</a></li>

<li><a href="Modules_(C%2B%2B)" title="Modules (C++)">Modules (C++)</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text">James Gosling, Bill Joy, Guy Steele, Gilad Bracha, <i>The Java Language Specification, Third Edition</i>, <style data-mw-deduplicate="TemplateStyles:r1238218222">

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</style><a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-321-24678-0</bdi>, 2005. In the Introduction, it is stated "Chapter 7 describes the structure of a program, which is organized into packages similar to the modules of Modula."</span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://docs.oracle.com/javase/specs/jls/se11/html/jls-7.html#jls-7.4.1">"Chapter 7. Packages and Modules"</a>. <i>docs.oracle.com</i><span class="reference-accessdate">. Retrieved <span class="nowrap">2021-12-10</span></span>.</cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-7.html#jls-7.5">"Chapter 7. Packages"</a>. Docs.oracle.com<span class="reference-accessdate">. Retrieved <span class="nowrap">2013-09-15</span></span>.</cite></span>

</li>

<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://docs.oracle.com/javase/tutorial/java/package/packages.html">"Creating and Using Packages (The Java™ Tutorials > Learning the Java Language > Packages)"</a>. <i>docs.oracle.com</i>.</cite></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html">"Controlling Access to Members of a Class (The Java™ Tutorials > Learning the Java Language > Classes and Objects)"</a>. <i>docs.oracle.com</i>.</cite></span>

</li>

<li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="http://www.oracle.com/technetwork/java/codeconventions-135099.html">Code Conventions for the Java Programming Language: 9. Naming Conventions</a></span>

</li>

<li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://docs.oracle.com/javase/specs/jls/se6/html/packages.html">"Packages"</a>. <i>docs.oracle.com</i>.</cite></span>

</li>

<li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20151208074800/http://cr.openjdk.java.net/~mr/jigsaw/ea/module-summary.html">"JDK Module Summary"</a>. <a href="Oracle_Corporation" title="Oracle Corporation">Oracle Corporation</a>. 2015-10-23. Archived from <a rel="nofollow" class="external text" href="http://cr.openjdk.java.net/~mr/jigsaw/ea/module-summary.html">the original</a> on 2015-12-08<span class="reference-accessdate">. Retrieved <span class="nowrap">2015-11-29</span></span>.</cite></span>

</li>

<li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://www.oracle.com/corporate/features/understanding-java-9-modules.html">"Understanding Java 9 Modules"</a>. <a href="Oracle_Corporation" title="Oracle Corporation">Oracle Corporation</a>. October 1, 2017<span class="reference-accessdate">. Retrieved <span class="nowrap">2022-10-04</span></span>.</cite></span>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="https://docs.oracle.com/en/java/javase/24/docs/api/">Java SE 24 API Javadocs</a></li></ul></div><div><div class="zim-footer">

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