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<b>Trail:</b> Learning the Java Language
<br /><b>Lesson:</b> Packages
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<div class="linkBHEAD"><a href="summary-package.html">Summary of Creating and Using Packages</a></div>
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<div id="PageTitle"><h1>Creating and Using Packages</h1></div>
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<!-- Creating and Using Packages -->
<p>To make types easier to find and use, to avoid naming conflicts, and to control access, programmers bundle groups of related types into packages.</p>
<div class="note"><hr /><strong>Definition:</strong> A <i>package</i> is a grouping of related types providing access protection and name space management. Note that <i>types</i> refers to classes, interfaces, enumerations, and annotation types. Enumerations and annotation types are special kinds of classes and interfaces, respectively, so <i>types</i> are often referred to in this
lesson
simply as <i>classes and interfaces</i>.
<hr /></div>
<p>The types that are part of the Java platform are members of various packages that bundle classes by function: fundamental classes are in <code>java.lang</code>, classes for reading and writing (input and output) are in <code>java.io</code>, and so on. You can put your types in packages too.</p>
<p>Suppose you write a group of classes that represent graphic objects, such as circles, rectangles, lines, and points. You also write an interface, <code>Draggable</code>, that classes implement if they can be dragged with the mouse.</p>
<div class="codeblock"><pre>
//<em>in the Draggable.java file</em>
public interface Draggable {
...
}
//<em>in the Graphic.java file</em>
public abstract class Graphic {
...
}
//<em>in the Circle.java file</em>
public class Circle extends Graphic
implements Draggable {
. . .
}
//<em>in the Rectangle.java file</em>
public class Rectangle extends Graphic
implements Draggable {
. . .
}
//<em>in the Point.java file</em>
public class Point extends Graphic
implements Draggable {
. . .
}
//<em>in the Line.java file</em>
public class Line extends Graphic
implements Draggable {
. . .
}
</pre></div>
<p>You should bundle these classes and the interface in a package for several reasons, including the following:<br /></p>
<ul>
<li>You and other programmers can easily determine that these types are related.</li>
<li>You and other programmers know where to find types that can provide graphics-related functions.</li>
<li>The names of your types won't conflict with the type names in other packages because the package creates a new namespace.</li>
<li>You can allow types within the package to have unrestricted access to one another yet still restrict access for types outside the package.</li>
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