a simple object oriented program
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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

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a simple object oriented program
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
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 objectSystem.out.fill(); // Error, no fill methodSystem.out.println(“howdy”); // OK
creating objects
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.

slide4
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)

www.java.sun.com provides class specs (see API specs)

slide5
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); printsjava.awt.Rectangle[x=5,y=10,width=45,height=15]
syntax variable definition
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
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
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
Uninitialized and Initialized Variables

Rectangle cerealBox;

Rectangle cerealBox = new Rectangle(5,10,20,30);

slide11
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.

writing a test program
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
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
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
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 methods1
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)

slide18
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)

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