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OOEDP Week 2 Lecture 2 1January 12

Introduction to Packages

Object Oriented and Event Driven Programming

Week 2 Lecture 2

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OOEDP Week 2 Lecture 2 2January 12

Objectives

In this chapter, we will

Introduce Packages

Discuss the location and naming of packages

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OOEDP Week 2 Lecture 2 3January 12

Packages in Java

What is a Package?

Why do we use Packages?

Where are Packages stored?

How are Packages located?

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OOEDP Week 2 Lecture 2 4January 12

Packages

Packages are used to group classes in a meaningful way

There are four main reasons why packages are used:

To avoid name conflicts

To simplify locating classes To distribute software conveniently

To control access to protected members of classes

The import statement is used to provide access to classesin a package:

import java.util.*;

import javax.swing.JOptionPane;

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OOEDP Week 2 Lecture 2 5January 12

Packages and Directories

In Java a package is a directory that contains thebytecode of the classes in the package

Packages can be hierarchical; that is packages within

packages This is reflected in the directory structure

For example I may wish to store the Card class in apackage called cardgames that is in a package calledmypackages in a package called pete

pete.mypackages.cardgames.Card

The class would be held in a directory structure such as:

C:\pete\mypackages\cardgames

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OOEDP Week 2 Lecture 2 6January 12

Packages and CLASSPATH

The directory that a package exists in does not have to beat the root level

However, the java tools need to be able to find the

package If the package is not at the root level then it is necessary

to set CLASSPATH to allow the tools to find the package

CLASSPATH is an environment variable

In windows

Modify the environment variables via control panel

In unix use the setenv command

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OOEDP Week 2 Lecture 2 7January 12

Adding Classes to a Package

The keyword package is used to indicate that a classbelongs to a certain package

For example:

package pete.mypackages.cardgamespublic class Card

{

....

The keyword package must be the first statement in a file

Apart from blank lines or comments

The package should be stored in an appropriate directory

Other classes can be added to the same package

One public class per file

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OOEDP Week 2 Lecture 2 8January 12

Naming Packages

Package name should be unique

The convention suggested is to use internet domainnames in reverse order as a package prefix

Remember internet domain names are unique For example:

Consider the domain www.fcet.staffs.ac.uk

This could be used as a package prefix:

uk.ac.staffs.fcet.pete.mypackages.cardgames

This makes directory organisation fun

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OOEDP Week 2 Lecture 2 9January 12

Summary

In this lecture we have:

Introduced Packages

Discussed the location and naming of packages