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Trail: Learning the Java Language
Lesson: Classes, Interfaces, and Packages

## Creating and Using Packages

> To make classes easier to find and use, to avoid naming conflicts, and to control access, programmers bundle groups of related classes into packages.

> > Definition: A package is a collection of related classes and interfaces that provides access protection and namespace management.

> The classes and interfaces that are part of the JDK are members of various packages that bundle classes by function: applet classes are in `java.applet`, I/O classes are in `java.io`, and the GUI widget classes are in `java.awt`. You can put your classes and interfaces in packages, too.

> Let's look at a set of classes and examine why you might want to put them in a package. Suppose you write a group of classes that represent a collection of graphics objects such as circles, rectangles, lines, and points. You also write an interface `Draggable` that classes implement if they can be dragged with the mouse by the user.

> >     in the Graphics.java file
> >     abstract class Graphic {
> >         . . .
> >     }

> >     in the Circle.java file
> >     public class Circle extends Graphic implements Draggable {
> >         . . .
> >     }

> >     in the Rectangle.java file
> >     public class Rectangle extends Graphic implements Draggable {
> >         . . .
> >     }

> >     in the Draggable.java file
> >     public interface Draggable {
> >         . . .
> >     }

> You should bundle these classes and the interface together in a package, for several reasons:
> - You and other programmers can easily determine that these classes and interfaces are related.
> - You and other programmers know where to find classes and interfaces that provide graphics-related functions.
> - The names of your classes won't conflict with class names in other packages because the package creates a new namespace.
> - You can allow classes within the package to have unrestricted access to each other, yet still restrict access for classes outside the package.

### Creating a Package

> You can easily create your own packages and put any number of class and interface definitions in them.

### Using Package Members

> To use the classes and interfaces defined in one package from within another package, you need to import the package. The classes and interfaces that you import must be declared public.

### Managing Source and Class Files

> This section shows you where to put your `java` and `.class` files so that the JDK tools can find your classes.

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