a Designing Program
Problem Specification and Decomposition. Determine the object's (things) that a program needs to work with (normally nouns in a problem specification) and the things it needs to do (normally verbs) by looking at the specification. Design a class that will represent a simple geometric rectangle with a given length and width. The definition of this class should make it possible to create rectangles of various dimensions and calculate their areas.
Java First Programs
August 24, 2006
Object Design
What is the object's role in the program? What data or information does it need to keep track of? What actions will it take or will be requested of it?
Data, Methods, and Algorithms
Values we store inside an object are normally referred to as instance variables and reflect the data that needs to be stored. Actions that the object is asked to perform become methods. The set of steps that the object will do when a method is called or invoked are an algorithm.
Some Simple Principles Before We Start
Programs should be easy to read and understand. Comments should be used to document and/or explain the program's code. Programs should be clear and should avoid tricks and unnecessarily obscure or convoluted code. Programs should be designed and written so that they are easy to maintain and change.
A Simple Program
/** * @author Jerry Heuring * * A first example using Java it only prints a * message but it is a start. */ public class Example1 extends Object { public void printMessage() { System.out.println("It's Alive!!!"); } public static void main (String arguments[]) { Example1 ourObject; ourObject = new Example1(); ourObject.print(); System.exit(0); } }
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The Pieces
/** * @author Jerry Heuring * * A first example using Java only prints a * message but it is a start. */
Defining the Class
This is the portion that declares a class. public class Example1 extends Object { Example1 is the name of our class. "extends Object" says it inherits from the object class in Java. If you don't say this it still inherits from the Object class.
This is a javadoc comment (starts with a /** and ends with an */). It identifies the author and the program's purpose. It's not required to run but we want it there for documentation purposes. The computer ignores comments but I don't.
"public class" says that this is a class that everybody (the public) can see.
}
Adding a Method
public void printMessage( ) { System.out.println("It's Alive!!!"); } This is a method. public says anybody can use it. void says it doesn't return anything. This tells the type of the result (if any) that the method returns. printMessage is the name of the method ( ) lists the items that the method needs to have passed to it. In this case, since the parenthesis are empty, nothing is passed. System.out.println( ) is a call to a method. The method belongs to the System.out object which is predefined. "It's Alive!!!" is a string constant (it can't be changed without rebuilding the program) and is passed to the println method.
Adding a Second Method
public static void main (String arguments[]) { Example1 ourObject; ourObject = new Example1(); ourObject.print(); System.exit(0); } This is an important routine. Java Applications start with the "main" routine. Without a main routine we couldn't run our application as Java wouldn't know where to start. It again returns nothing is and visible to everybody (public and void) The static says there is only one copy of this routine no matter how many Example1 objects we make. More on this later. The "String arguments[]" in the parenthesis says that the main program expects an array (or list) of Strings to be passed in. We will ignore this for right now.
The Internal Items
Example1 ourObject; ourObject = new Example1(); ourObject.print(); System.exit(0);
Running the Program
When we compile and run the program it prints out the line: It's Alive!!!
The statements in the method do a number of things: Example1 ourObject; creates a variable, ourObject, which will hold a reference to an object of the type Example1. ourObject = new Example1(); creates or instantiates a new object using the Example1 class as a blueprint. The reference to this object is then stored in ourObject. ourObject.print(); calls the print method belonging to the object we just created. System.exit(0); calls a method to exit the program.
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Classes
Notice everything is inside of a class right now.
Everything in Java must be contained in classes. Each class is usually placed in a separate file. The filename must match the class name.
Constructors
Constructors are used to initialize an object into a (hopefully) known state. They may do some computation There may be more than one constructor as long as the number or types of the arguments to the constructor are different. They all have the same name as the class.
When Is the Constructor Called?
A common question is when is a constructor actually called?
When an object is instantiated (big word may want to think of it as created).
Simple Demonstration Program
/** * @author Jerry Heuring * Created May 27, 2004 * * @version 1.0 * * Bugs * * Revisions * * This class is a simple demonstration to demonstrate when a * constructor is called. It also tries to demonstrate the * need for instantiating (creating) an object in the main * program. */ public class ConstructorDemo {
When is an object instantiated?
Usually with a "new objectT...