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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:
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.
The next module in this collection will address the special case of Array Objects.
This section contains a variety of miscellaneous information.
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.
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-
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