2.34 Jb0270: java oop: packages

Revised: Tue Mar 29 10:38:11 CDT 2016

This page is included in the following Books:

• Programming Fundamentals with Java
• Object-Oriented Programming (OOP) with Java

Table of contents

• Preface
  • Viewing tip
    • Listings
• Introduction
• Classpath environment variable
• Developing your own packages
  • The package directive
  • The import directive
  • Compiling programs with the package directive
  • Sample program
• Run the program
• Looking ahead
• Miscellaneous

Preface

This module explains the concept of packages and provides a sample program that illustrates the concept.

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the listings while you are reading about them.

Listings

• Listing 1 . File: Package00.java.
• Listing 2 . File Package01.java.
• Listing 3 . File Package02.java.
• Listing 4 . File: CompileAndRun.bat.

Introduction

Before you can understand much about packages, you will need to understand the classpath environment variable , so that is where I will begin the discussion.

After learning about the classpath environment variable, you will learn how to create your own packages.

Classpath environment variable

The purpose of the classpath environment variable is to tell the JVM and other Java applications (such as the javac compiler) where to find class files and class libraries. This includes those class files that are part of the JDK and class files that youmay create yourself.

I am assuming that you already have some familiarity with the use of environment variables in your operating system. All of thediscussion in this module will be based on the use of a generic form of Windows. (By generic, I mean not tied to any particular version of Windows.) Hopefully you will be able to translate the information to your operating system if you areusing a different operating system.

There are usually a fairly large number of environment variables installed on a machine at any give time. If you would like to see what kind ofenvironment variables are currently installed on your machine, bring up acommand-line prompt and enter the command set . This should cause the names of several environment variables, along with their settings to bedisplayed on your screen.

While you are at it, see if any of those items begin with CLASSPATH= . If so, you already have a classpath environment variable set on yourmachine, but it may not contain everything that you need.

I am currently using a Windows 7 operating system and no classpath environment variable is set on it. I tend to use the -cp switch option (see Listing 4 ) in the JDK to set the classpath on a temporary basis when I need it to be set.

Rather than trying to explain all of the ramifications regarding the classpath, I will simply refer you to an Oracle document on the topic titled Setting the class path .