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Revised: Mon Mar 28 11:44:44 CDT 2016
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This module takes a preliminary look at the complexity of OOP by examining some aspects of the System and PrintStream classes.
I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the listings while you are reading about them.
This lesson introduces you to the use of the System and PrintStream classes in Java. This is our first introduction to the complexity that can accompany the OOP paradigm. It gets a little complicated, so you might need to pay special attention to the discussion.
What does the main method do?
The main method in the controlling class of a Java application controls the flow of the program.
The main method can also access other classes along with the variables and methods of those classes and of objects instantiated from thoseclasses.
The hello1 Application
Listing 1 shows a simple Java application named hello1 .
(By convention, class definitions should begin with an upper-case character. However, the original version of this module was written andpublished in 1997, before that convention was firmly established.)
| Listing 1 . The program named hello1. |
|---|
/*File hello1.java Copyright 1997, R.G.Baldwin
**********************************************************/class hello1 { //define the controlling class
//define main methodpublic static void main(String[] args){//display text string
System.out.println("Hello World");}//end main
}//End hello1 class. No semicolon at end of Java class. |
Does this program Instantiate objects?
This is a simple example that does not instantiate objects of any other class.
Does this program access another class?
However, it does access another class. It accesses the System class that is provided with the Java development kit. (The System class will be discussed in more detail in a future module.)
The variable named out
The variable named out , referred to in Listing 1 as System.out , is a class variable of the System class. (A class variable is a variable that is declared to be static.)
Recall that a class variable can be accessed without a requirement to instantiate an object of the class. As is the case with all variables, the classvariable must be of some specific type.
Primitive variables vs. reference variables
A class variable may be a primitive variable , which contains a primitive value, or it may be a reference variable , which contains a reference to an object.
(I'll have more to say about the difference between primitive variables andreference variables in a future module.)
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