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Learn about abstract methods, abstract classes, and overridden methods. You will also learn about overriding the toString method.

Revised: Sat Apr 02 20:33:36 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 Figures and Listings while you are reading about them.

Figures

  • Figure 1 . Program output on the command line screen.
  • Figure 2 . Default behavior of the toString method.
  • Figure 3 . More on the default behavior of the toString method.

Listings

  • Listing 1 . Source code for class Prob04.
  • Listing 2 . Beginning of the class named Prob04MyClass.
  • Listing 3 . Override the abstract getData method.
  • Listing 4 . Override the toString method.
  • Listing 5 . Complete program listing.

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:

  • Extending an abstract class.
  • Parameterized constructor.
  • Defining an abstract method in the superclass and overriding it in a subclass.
  • Overridden toString method.

Program specifications

Write a program named Prob04 that uses the class definition shown in Listing 1 to produce the output on the command-line screen shown in Figure 1 .

Figure 1 . Program output on the command line screen.
Prob04 DickBaldwin 9595

Pseudo random data

Because the program generates and uses a pseudo random data value each time it is run, the actual values displayed in the last two lines of Figure 1 will differ from one run to the next. However, in all cases, the two values 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 Prob04 given below.

Discussion and sample code

Will explain in fragments

I will explain this program in fragments. A complete listing is provided in Listing 5 near the end of the module.

I will begin with the driver class named Prob04 , which is shown in its entirety in Listing 1 .

Listing 1 . Source code for class Prob04.
import java.util.*; abstract class Prob04{public static void main(String[] args){Random generator = new Random(new Date().getTime()); int randomNumber = (byte)generator.nextInt();Prob04 objRef = new Prob04MyClass(randomNumber);System.out.println(objRef); System.out.println(objRef.getData());System.out.println(randomNumber); }//end main//Declare the signature of an abstract class. public abstract int getData();}//end class Prob04

The import directive

The import directive at the beginning of Listing 1 is required because the program requires access to the Random class and the Date class, both of which are defined in the java.util package.

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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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