The Essence of OOP using Java: Static Members…

Class members and instance members

Class variables and class methods are declared static (declaring a member static in the class definition causes to be called a class member).

Instance variables and instance methods are not declared static.

Each object has its own copy ...

Every object instantiated from a given class has its own copy of each instance variable declared in the class definition. (Instance variables are not shared among objects.)

Every object instantiated from a given class acts like it has its own copy of every instance method declared in the class definition. (Although instance methods are actually shared among objects in order to reduce the amount of memory required, they are shared in such a way that they don't appear to be shared.)

Every object shares ...

Every object instantiated from a given class shares the same single copy of each class variable declared in the class definition. Similarly, every object instantiated from a given class shares the same copy of each class method.

Accessing an instance member

An instance variable or an instance method can only be accessed by using a reference to the object that owns it. Even then, it may or may not be accessible depending on the access modifier assigned by the programmer.

Accessing a class member

The single shared copy of a class variable or a class method can be accessed in either of two ways:

• Via a reference to any object instantiated from the class
• By simply joining the name of the class to the name of the class variable or the class method

Again, the variable or method may or may not be accessible, depending on the access modifiers assigned by the programmer.

When to use class variables

It is very often appropriate to use final static variables, as constants in your programs. It is rarely, if ever, appropriate to use non-final static variables in your programs. The use of non-final static variables should definitely be minimized.

When to use static methods

It is often appropriate to use static methods in your programs, provided there is no requirement for the method to remember anything from one invocation to the next. Static methods should be self-contained.

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

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