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Creating and Using Packages

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

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Definition:  A package is a collection of related types providing access protection and namespace management. Note that types refers to classes, interfaces, enums and annotations. For more information on enums, see the section on Enumerated Types.

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The types 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 types 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 types are related.
• You and other programmers know where to find types that provide graphics-related functions.
• The names of your types won't conflict with types names in other packages, because the package creates a new namespace.
• You can allow types within the package to have unrestricted access to one another yet still restrict access for types outside the package.

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