PACKAGES

java packages

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/*in the Draggable.java file */
public interface Draggable {      ...  }
/*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 {      . . .  }
 
Package
You should bundle these classes and the interface in a package for several reasons, including the following:
You and other programmers can easily determine that these types are related.
You and other programmers know where to find types that can provide graphics-related functions.
PackageThe names of your types won't conflict with the type 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.
 
Creating a Package
To create a package, you choose a name for the package (naming conventions are discussed in the next section) and put a package statement with that name at the top of every source file that contains the types (classes, interfaces, enumerations, and annotation types) that you want to include in the package.
The package statement (for example, package graphics;) must be the first line in the source file. There can be only one package statement in each source file, and it applies to all types in the file.

Note: If you put multiple types in a single source file, only one can be public, and it must have the same name as the source file. For example, you can define public class Circle in the file Circle.java, define public interface Draggable in the file Draggable.java, define public enum Day in the file Day.java, and so forth. 
You can include non-public types in the same file as a public type (this is strongly discouraged, unless the non-public types are small and closely related to the public type), but only the public type will be accessible from outside of the package. All the top-level, non-public types will be package private.

Definition: A package is a grouping of related types providing access protection and name space management. Note that types refers to classesinterfaces, enumerations, and annotation types. Enumerations and annotation types are special kinds of classes and interfaces, respectively, so types are often referred to in this lesson simply as classes and interfaces.

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.Package

Suppose you write a group of classes that represent 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.