WebReference.com - Part 4 of chapter 5 from Beginning Java 2 SDK 1.4 Edition, Wrox Press Ltd (3/8)

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Beginning Java 2 SDK 1.4 Edition

Adding Classes from a Package to Your Program

Assuming they have been defined with the public keyword, you can add all or any of the classes in a package to the code in your program by using an import statement. You can reference the classes that you make available to your program using the import statement just by using the class names. For example, to make available all the classes in the package Geometry.Shapes3D to a source file, you just need to add the following import statement to the beginning of the file:

import Geometry.Shapes3D.*;   // Include all classes from this package

The keyword import is followed by the specification of what you want to import. The wildcard *, following the period after the package name, selects all the classes in the package, rather like selecting all the files in a directory. Now you can refer to any public class in the package just by using the class name. Again, the names of other classes in your program must be different from the names of the classes in the package.

If you want to add a particular class rather than an entire package, you specify its name explicitly in the import statement:

import Geometry.Shapes3D.Sphere;  // Include the class Sphere

This includes only the Sphere class into the source file. By using a separate import statement for each individual class from the package, you can ensure that your source file only includes the classes that you need. This reduces the likelihood of name conflicts with your own classes, particularly if you are not fully familiar with the contents of the package and it contains a large number of classes.

Note that the * can only be used to select all the classes in a package. You can't use Geometry.* to select all the packages in the directory Geometry.

Packages and Names in Your Programs

A package creates a self-contained environment for naming your classes. This is the primary reason for having packages in Java. You can specify the names for classes in one package without worrying about whether the same names have been used elsewhere. Java makes this possible by treating the package name as part of the class name--actually as a prefix. This means that the class Sphere in the package Geometry.Shapes3D has the full name Geometry.Shapes3D.Sphere. If you don't use an import statement to incorporate the class in your program, you can still make use of the class by referring to it using its full class name. If you needed to do this with the class Sphere, you might declare a variable with the statement:

Geometry.Shapes3D.Sphere ball = new Geometry.Shapes3D.Sphere(10.0, 1.0, 1.0, 1.0);

While this is rather verbose, and certainly doesn't help the readability of the program, it does ensure there will be no conflict between this class and any other Sphere class that might be part of your program. You can usually contrive that your class names do not conflict with those in the commonly used standard Java packages, but in cases where you can't manage this, you can always fall back on using fully qualified class names. Indeed, there are occasions when you have to do this. This is necessary when you are using two different classes from different packages that share the same basic class name.

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Created: July 29, 2002
Revised: July 29, 2002

URL: https://webreference.com/programming/java/beginning/chap5/4/3.html