A Simple Object Oriented Program public class Simple { public static void main (String [] args) { System.out.println(“howdy”); } System.out is an object of class PrintStream, provided by the Java library println is a method of the PrintStream class

Objects and Classes  Object: entity that contains data and can performs actions in a program (by invoking methods)  Class: describes a type of object, a class must be defined in order to ‘create’ an object  So, each object belongs to a class  Class determines legal methods for an object System.out.fill(); // Error, no fill method System.out.println(“howdy”); // OK

Creating Objects The System.out object has already been created, and already exists for your use in a Java program. Typically, you must create an object before you can use it. In order to create an object, a class must exist which ‘describes’ this type of object. To use this class, you must be familiar with the creation methods (constructors) it provides. A constructor method always has the same name as the class.

One class which already exists for your use in the Java library is the Rectangle class. Let’s create a Rectangle object……… Two of the constructors available with the Rectangle class: Rectangle() Rectangle(int x, int y, int width, int height) provides class specs (see API specs)

Declare a rectangle reference variable: Rectangle myrec; Construct a rectangle: myrec = new Rectangle(5, 10, 20, 30); Adjust the size of this Rectangle object by calling the setsize method of the Rectangle class: myrec.setsize(45,15); Print the Rectangle object System.out.println(myrec); prints java.awt.Rectangle[x=5,y=10,width=45,height=15]

Attributes of a Rectangle Object

Syntax: Variable Definition TypeName variableName; TypeName variableName = expression; –Example: Rectangle myrec; Rectangle box = new Rectangle(); –Purpose: To define a new variable of a particular type and optionally supply an initial value

Syntax: Object Construction new ClassName(parameters) –Example: new Rectangle(5, 10, 20, 30) new Car(“red”, “Ford”,”Pinto”, 4.5) –Purpose: To construct a new object, initialize it with the construction parameters, and return a reference to the constructed object.

Object Variables Declare Rectangle crispyCrunchy; Declare and initialize: Rectangle cerealBox = new Rectangle(5, 10, 20, 30);  Once you have an object, you can apply methods: cerealBox.translate(15, 25); crispyCrunchy.translate(10,5); //error !!  Share objects: r = cerealBox;

Uninitialized and Initialized Variables Rectangle cerealBox; Rectangle cerealBox = new Rectangle(5,10,20,30);

These variables are references (like pointers) to objects, SO: Rectangle r; r = cerealBox; is NOT the same as: r = new Rectangle(5,10,20,30); In the first case, two variables will refer to the SAME object, in the second, an additional object is created and referred to by crispCrunchy.

Two Object Variables Referring to the Same Object

Writing a Test Program  Invent a new class, say MoveTest  Supply a main method (which makes this an application class)  Place instructions inside the main method  Import any library classes you need by specifying the package and class name: import java.awt.Rectangle;  You don't need to import classes in the java.lang package such as String and System

Syntax : Importing a Class from a Package import packageName.ClassName ; –Example: import java.awt.Rectangle; –Purpose: To import a class from a package for use in a program

File MoveTest.java import java.awt.Rectangle; public class MoveTest { public static void main(String[] args){ // declare and create the rectangle Rectangle cerealBox = new Rectangle(5,10,20,30); // move the rectangle cerealBox.translate(15, 25); // print the moved rectangle System.out.println(cerealBox); }

Using Class Methods In order to call a class method successfully, you must examine it’s signature: // the following bold signature indicates how to use this // method of the Rectangle class public boolean contains (int x, int y) { //method body }

Using Class Methods  access specifier (such as public)  return type (such as String or void)  method name (such as contains or setSize)  list of parameters (number, type and purpose of parameters needed in call )

public boolean contains (int x, int y) { //method body } This method is ‘callable’, because its access specifier is public This method will return a true or false result, so the call should occur within an expression The method is called using the name ‘contains’ the call must provide 2 arguments Call: myrec.contains(6,7)