Lecture 14 Objects and Classes Object-oriented programming (OOP) - - PowerPoint PPT Presentation

Lecture 14

Objects and Classes

Object-oriented programming (OOP)

• We’re always looking for ways to standardize, organize

and simplify our code. Objects can help

• We use objects to represent things in the real world -

like a student, a receipt, a hand, etc

• Objects have a unique identity, state, and behavior.
• Especially useful when you have multiple copies of the

same type

Object-oriented programming (OOP)

• Identity = variable name, location in memory
• State = data fields (variables)
• Behavior = methods

Object example: ball

• Identity =
• State =
• Behavior =

redBall ballColor, radius, position fall(), bounce()

Creating and using

• bjects
• Objects are created using a class
• A class is like a template for the object
• A class stores the data fields and methods for the object
• An object is often referred to as an instance of a class.
• So the variable redBall would be of type Ball, for example
• Class is to Object/Instance, as Lasagne Recipe is to an actual

lasagne

Creating and using

• bjects
• Classes also have special methods, constructors
• Constructors are used when making a new object.
• These are useful when you want to set default values, for

example

Circle Example

public class Circle { /** The radius of this circle */ double radius = 1; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double getArea() { return radius * radius * Math.PI; } /** Return the perimeter of this circle */ double getPerimeter() { return 2 * radius * Math.PI; } /** Set new radius for this circle */ void setRadius(double newRadius) { radius = newRadius; } }

Circle Example

Constructors - they’re weird

• Must have the same name as the class
• NO return type! Not even void
• Invoked using new when the object is created
• Can be overloaded

Constructors - they’re weird

/** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Construct a circle object */ Circle(double newRadius, int xIn, int yIn){ radius = newRadius; xPos = xIn; yPos = yIn; }

Reference variables

• Objects are created and accessed using a reference

variable

• This is a variable that is used to refer to a specific

instance of a class

ClassName objectRefVar = new ClassName(); Circle myCircle = new Circle();

Instance variables

• Variables in the class
• can be accessed using the dot operator (e.g.

redBall.radius)

• If it is a reference value (not a primitive type) and doesn’t

have a default value, it will be null. Good example are strings

• null is a literal value, similar to true or false

Test class for circle

public class TestCircle { public static void main(String[] args) { Circle circle1 = new Circle(); System.out.println("The area of the circle of radius " + circle1.radius + " is " + circle1.getArea()); Circle circle2 = new Circle(25); System.out.println("The area of the circle of radius " + circle2.radius + " is " + circle2.getArea()); circle2.setRadius(100); System.out.println("The area of the circle of radius " + circle2.radius + " is " + circle2.getArea()); } }

Tons of classes available in java already!

• For example, Date, Random, Scanner, etc.
• Objects of these types are created and used in the exact

same way

Date

import java.util.*; public class DateTesting { public static void main(String[] args) { Date date = new Date(); System.out.println(date.toString()); } }

Random

import java.util.*; public class RandomTesting { public static void main(String[] args) { Random rand1 = new Random(3); Random rand2 = new Random(3); Random rand3 = new Random(); for (int i = 0; i < 100; i++){ System.out.println("rand1: " + rand1.nextInt(10)); System.out.println("rand2: " + rand2.nextInt(10)); } // For a new random integer within a range int max = 10; int min = 5; int randomNum = rand3.nextInt((max - min) + 1) + min; System.out.println(randomNum); } }