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Learn about indirection, array objects, and casting.

Revised: Sun Apr 03 09:25:24 CDT 2016

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Table of contents

Preface

This module is one of a series of modules designed to teach you about Object-Oriented Programming (OOP) using Java.

Viewing tip

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

Figures

  • Figure 1 . Program output on command line screen.

Listings

Preview

The program that I will explain in this module produces no graphics and does not require the use of Ericson's media library.

OOP concepts

The program illustrates the following OOP concepts among others:

  • Multiple levels of indirection
  • A one-element array of type Object
  • Storing a reference to an object in an array element as type Object
  • An anonymous object
  • Passing a reference to a subclass object as type Object
  • Downcasting an incoming object reference to access a method

Program specifications

Write a program named Prob05 that uses the class definition shown in Listing 1 to produce an output similar to that shown in Figure 1 on the command-line screen.

Figure 1 . Program output on command line screen.
Prob05 DickBaldwin -28-28

A random value

Because the program generates and uses a random data value, the actual values displayed will differ from one run to the next. However, in all cases, the twovalues shown in Figure 1 must match.

New classes

You may define new classes as necessary to cause your program to behave as required, but you may not modify the class definition for the class named Prob05 which begins in Listing 1 .

Discussion and sample code

Will explain in fragments

I will explain this program in fragments. A complete listing is provided in Listing 6 near the end of the module.

I will begin with the driver class named Prob05 , which begins in Listing 1 .

Listing 1 . Beginning of the Prob05 class.
import java.util.*; class Prob05{public static void main(String[] args){Random generator = new Random(new Date().getTime()); int randomNumber = (byte)generator.nextInt();Object[] objRef = {new Prob05MyClassA(randomNumber)};

Everything in Listing 1 should be familiar to you with the possible exception of the last statement, which I will explain shortly.

Characteristics of arrays in Java

Before explaining that statement, however, I will discuss some of the important characteristics of array objects in Java. A list of suchcharacteristics follows in no particular order:

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Source:  OpenStax, Object-oriented programming (oop) with java. OpenStax CNX. Jun 29, 2016 Download for free at https://legacy.cnx.org/content/col11441/1.201
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