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Java How to Program, 6/e

ISBN:
0-13-148398-6
© 2005
pages: 1576
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The Java class libraries are divided into packages of related classes and interfaces. Most Java programs you build reuse existing packages, such as the Java class libraries, third party libraries or libraries developed within your organization. This tutorial shows how create your own packages of classes for reuse or distribution. The tutorial is intended for students or professionals who are alreadyfamiliar with building Java classes.

Download the code for this tutorial here.

[Note: This tutorial is an excerpt (Section 8.16) of Chapter 8, Class and Objects: A Deeper Look, from our textbook Java How to Program, 6/e. This tutorial may refer to other chapters or sections of the book that are not included here. Permission Information: Deitel, Harvey M. and Paul J., JAVA HOW TO PROGRAM, ©2005, pp.390-396. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.]

8.16 Time Class Case Study: Creating Packages (Continued)

    Line 3 is known as a single-type-import declaration—that is, the import declaration specifies one class to import. When your program uses multiple classes from the same package, you can import those classes with a single import declaration. For example, the import declaration

import java.util.*; // import classes from package java.util

uses an asterisk (*) at the end of the import declaration to inform the compiler that all classes from the java.util package are available for use in the program. This is known as a type-import-on-demand declaration. Only the classes from package java.util that are used in the program are loaded by the JVM. The preceding import allows you to use the simple name of any class from the java.util package in the program. Throughout this book, we use single-type-import declarations for clarity.

Common Programming Error
Common Programming Error 8.12
Using the import declaration import java.*; causes a compilation error. You must specify the exact name of the package from which you want to import classes.

Specifying the Classpath During Compilation

When compiling Time1PackageTest, javac must locate the .class file for Time1 to ensure that class Time1PackageTest uses class Time1 correctly. The compiler uses a special object called a class loader to locate the classes it needs. The class loader begins by searching the standard Java classes that are bundled with the JDK. Then it searches for optional packages. Java provides an extension mechanism that enables new (optional) packages to be added to Java for development and execution purposes. [Note: The extension mechanism is beyond the scope of this book. For more information, visit java.sun.com/j2se/5.0/docs/guide/extensions.] If the class is not found in the standard Java classes or in the extension classes, the class loader searches the classpath, which contains a list of locations in which classes are stored. The classpath consists of a list of directories or archive files, each separated by a directory separator—a semicolon (;) on Windows or a colon (:) on UNIX/Linux/Mac OS X. Archive files are individual files that contain directories of other files, typically in a compressed format. For example, the standard classes used by your programs are contained in the archive file rt.jar, which is installed with the JDK. Archive files normally end with the .jar or .zip file-name extensions. The directories and archive files specified in the classpath contain the classes you wish to make available to the Java compiler and the JVM.

    By default, the classpath consists only of the current directory. However, the classpath can be modified by

1. providing the -classpath option to the javac compiler or

2. setting the CLASSPATH environment variable (a special variable that you define and the operating system maintains so that applications can search for classes in the specified locations).

For more information on the classpath, visit java.sun.com/j2se/5.0/docs/tooldocs/tools.html. The section entitled “General Information” contains information on setting the classpath for UNIX/Linux and Windows.

Common Programming Error
Common Programming Error 8.13
Specifying an explicit classpath eliminates the current directory from the classpath. This prevents classes in the current directory (including packages in the current directory) from loading properly. If classes must be loaded from the current directory, include a dot (.) in the classpath to specify the current directory.
Software Engineering Observation
Software Engineering Observation 8.16
In general, it is a better practice to use the -classpath option of the compiler, rather than the CLASSPATH environment variable, to specify the classpath for a program. This enables each application to have its own classpath.
Error-Prevention Tip
Error-Prevention Tip 8.3
Specifying the classpath with the CLASSPATH environment variable can cause subtle and difficult-to-locate errors in programs that use different versions of the same package.

    For the example of Fig. 8.18 and Fig. 8.19, we did not specify an explicit classpath. Thus, to locate the classes in the com.deitel.sjhtp6.ch08 package from this example, the class loader looks in the current directory for the first name in the package—com. Next, the class loader navigates the directory structure. Directory com contains the subdirectory deitel. Directory deitel contains the subdirectory sjhtp6. Finally, directory sjhtp6 contains subdirectory ch08. In the ch08 directory is the file Time1.class, which is loaded by the class loader to ensure that the class is used properly in our program.

Specifying the Classpath When Executing an Application

When you execute an application, the JVM must be able to locate the classes used in that application. Like the compiler, the java command uses a class loader that searches the standard classes and extension classes first, then searches the classpath (the current directory by default). The classpath for the JVM can be specified explicitly by using either of the techniques discussed for the compiler. As with the compiler, it is better to specify an individual program’s classpath via command-line options to the JVM. You can specify the classpath in the java command via the -classpath or -cp command-line options, followed by a list of directories or archive files separated by semicolons (;) on Microsoft Windows or by colons (:) on UNIX/Linux/Mac OS X. Again, if classes must be loaded from the current directory, be sure to include a dot (.) in the classpath to specify the current directory.

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