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Creating and Using Packages

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

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

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The classes and interfaces that are part of the Java platform are members of various packages that bundle classes by function: fundamental classes are in java.lang, classes for reading and writing (input and output) are in java.io, and so on. 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 that you write a group of classes that represent a collection of graphic 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 Graphic.java file
public 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 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 wont 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 one another yet still restrict access for classes outside the package.

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