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Summary

Added complexity

The existence of static members tends to break up the simple OOP structures that I have discussed in previous modules in this collection.

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

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

The class named Class

I discussed the class named Class and how a conceptual object of type Class exists in memory following a reference to a specific class in the program code.

The Class object represents the referenced class in memory, and contains the static variables and static methods belonging to that class. (It contains some other information as well, such as the name of the superclass.)

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 ofmemory 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 objectinstantiated 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 accessibledepending 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 call to thenext. Static methods should be self-contained.

What's next?

The next module in this collection will address the special case of Array Objects.

Miscellaneous

This section contains a variety of miscellaneous information.

Housekeeping material
  • Module name: Java OOP: Static Members
  • File: Java1620.htm
  • Published: 04/24/02
Disclaimers:

Financial : Although the Connexions site makes it possible for you to download a PDF file for thismodule at no charge, and also makes it possible for you to purchase a pre-printed version of the PDF file, you should beaware that some of the HTML elements in this module may not translate well into PDF.

I also want you to know that, I receive no financial compensation from the Connexions website even if you purchase the PDF version of the module.

In the past, unknown individuals have copied my modules from cnx.org, converted them to Kindle books, and placed them for sale on Amazon.com showing me as the author. Ineither receive compensation for those sales nor do I know who does receive compensation. If you purchase such a book, please beaware that it is a copy of a module that is freely available on cnx.org and that it was made and published withoutmy prior knowledge.

Affiliation : I am a professor of Computer Information Technology at Austin Community College in Austin, TX.

Complete program listing

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

Listing 13 . Complete program listing.
/*File MyClass01.java Copyright 2002, R.G.BaldwinThis program illustrates static members of a class. Output is:Static variable Mon Sep 17 09:52:27 CDT 2001Instance variable Mon Sep 17 09:52:32 CDT 2001Static variable Mon Sep 17 09:52:27 CDT 2001Instance variable Mon Sep 17 09:52:37 CDT 2001Static 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 variableMyClass01 ref1 = new MyClass01(); System.out.println();//blank lineSystem.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 secondstry{ Thread.currentThread().sleep(5000);}catch(InterruptedException e){} //Instantiate another objectMyClass01 ref2 = new MyClass01(); System.out.println();//blank lineSystem.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

-end-

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
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what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
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