The Essence of OOP using Java: Static Members…

What's Next?

The next lesson in this miniseries will address the special case of Array Objects.

Complete Program Listing

A complete listing of the sample program is shown in Listing 13.

/*File MyClass01.java

Copyright 2002, R.G.Baldwin



This program illustrates static 

members of a class.  Output is:

  

Static variable

Mon Sep 17 09:52:27 CDT 2001



Instance variable

Mon Sep 17 09:52:32 CDT 2001

Static variable

Mon Sep 17 09:52:27 CDT 2001





Instance variable

Mon Sep 17 09:52:37 CDT 2001

Static variable

Mon Sep 17 09:52:27 CDT 2001

**************************************/

import java.util.Date;

class MyClass01{

  static Date v1 = new Date();

  Date v2 = new Date();



  public static void main(

                        String[] args){

    //Display static variable

    System.out.println(

                    "Static variable");

    System.out.println(MyClass01.v1);

    

    //Delay for five seconds

    try{

    Thread.currentThread().sleep(5000);

    }catch(InterruptedException e){}

    

    //Instantiate an object and 

    // display instance variable

    MyClass01 ref1 = new MyClass01();

    System.out.println();//blank line

    System.out.println(

                  "Instance variable");

    System.out.println(ref1.v2);

    

    //Now, display the static

    // variable using object reference

    System.out.println(

                    "Static variable");

    System.out.println(ref1.v1);

    

    System.out.println();//blank line

    

    //Delay for five seconds

    try{

    Thread.currentThread().sleep(5000);

    }catch(InterruptedException e){}

    

    //Instantiate another object 

    MyClass01 ref2 = new MyClass01();

    System.out.println();//blank line

    System.out.println(

                  "Instance variable");

    System.out.println(ref2.v2);

    

    //Now, display the same static

    // variable using object reference

    System.out.println(

                    "Static variable");

    System.out.println(ref2.v1);

  }//end main

}//end class MyClass01

//===================================//



Listing 13

---

Preface

This lesson is one of a series of lessons designed to teach you about the essence of Object-Oriented Programming (OOP) using Java. The first lesson in the group was entitled The Essence of OOP Using Java, Objects, and Encapsulation. That lesson, and each of the lessons following that one, has provided explanations of certain aspects of the essence of Object-Oriented Programming using Java. The previous lesson was entitled The Essence of OOP using Java, Polymorphism and Interfaces, Part 2.

Necessary and significant aspects

This miniseries will describe and discuss the necessary and significant aspects of OOP using Java. If you have a general understanding of computer programming, you should be able to read and understand the lessons in this miniseries, even if you don't have a strong background in the Java programming language.

Supplementary material

I recommend that you also study the other lessons in my extensive collection of online Java tutorials. You will find those lessons published at Gamelan.com. However, as of the date of this writing, Gamelan doesn't maintain a consolidated index of my Java tutorial lessons, and sometimes they are difficult to locate there. You will find a consolidated index at Baldwin's Java Programming Tutorials.

Preview

Static members

There is another aspect of OOP in Java that I have avoided up to this point in the discussion: static variables and static methods.

Tends to complicate ...

I have avoided this topic because, while not particularly difficult, the existence of static members tends to break up the simple structures that I have discussed in previous lessons in this miniseries.

While static members can be useful in some situations, the existence of static members tends to complicate the overall object-oriented structure of Java.

Avoid overuse of static members

Furthermore, the overuse of static members can lead to problems similar to those experienced in languages like C and C that support global variables and global functions.

When to use static members

I will discuss the use of static members in this lesson, and will provide some guidelines for their use.

The class named Class

I will also introduce the class named Class and discuss how it enters into the use of static variables and methods.

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