Object Oriented Programming Concepts

• OOP – reasoning about a program as a set of objects rather than as a set of actions

• Object – a programming entity that contains state and behavior

• State – set of values (internal data) stored in an object

• Behavior – set of actions an object can perform, often reporting or modifying its state

class.ppt 1

Objects

• Objects are not complete programs

• Objects are components with distinct roles and responsibilities

• Java contains over 3000 classes of objects– String, Point, Scanner, File

• Used by clients

class.ppt 2

class.ppt 3

Class• State

– Fields: variable inside an object that makes up part of its internal state

– Using data types that already exist– Implicit initialization

• Behavior– Instance Methods: a method inside an object that operates

on that object– Mutator: an instance method that modifies the object’s

internal state (name begins with set)– Accessor: an instance method that provides information

about the state of an object without modifying it (name often begins with get or is)

• Make each class its own file

public class Point

{

int x;

int y;

public double distanceFromOrigin()

{

return Math.sqrt(x*x + y*y);

}

//shifts this points location by the given amount

public void shift (int dx, int dy)

{

x += dx;

y += dy;

}

}

class.ppt 4

class.ppt 5

Declaring objects

• Done in client • Point p1; // no space allocated• Example: • Point point1 = new Point(); //allocates space

Point point2 = new Point();• Note: The declarations create 2 new objects

of the Point class. Each object has its own copies of  x and y.

class.ppt 6

Class Objects or Class Instances

Point point1;

x 5

y 30

Point point2;

x 17

y 58

Client

• Point point1 = new Point();

• point1.x = 7;

• point1.y = 3;

• point1.shift(2,-1);

• System.out.println(p1.distanceFromOrigin());

class.ppt 7

Constructor

• A special method that initializes the state of new objects as they are created

• Same name as class

• No type

• Executed when an instance is made

class.ppt 8

Constructors

• If you create an alternate constructor, then you must (also) create the default constructor. If you do not include any constructor in a class, then Java automatically provides the default constructor.

class.ppt 9

Constructorspublic Point()

{

x = 0;

y = 0;

}

public Point(int initialX, int initialY)

{

x = initialX;

y = initialY;

} class.ppt 10

public class Point

{int x;

int y;

public Point() //default constructo

{

x = 0;

y = 0;

}

public Point(int initialX, int initialY) // constructor

{

x = initialX;

y = initialY;

}

//returns the distance between this point and (0,0)

public double distanceFromOrigin()

{

return Math.sqrt(x*x + y*y);

}

//shifts this points location by the given amount

public void shift (int dx, int dy)

{

x += dx;

y += dy;

}

}

class.ppt 11

Client code

• Point p1 = new Point(); //calls default

• Point p2 = new Point(6,1);

class.ppt 12

Client codepublic class PointMain{

public static void main(String[] args) {

Point p1 = new Point(7,4);

Point p2 = new Point(6,1);

//print each point and its distance from the origin

System.out.println("p1 is (" + p1.x + ", " + p1.y + ")");

System.out.println("distance from origin = " + p1.distanceFromOrigin());

System.out.println("p2 is (" + p2.x + ", " + p2.y + ")");

System.out.println("distance from origin = " + p2.distanceFromOrigin());

//shift

p1.shift(11,6);

p2.shift(1,7);

//print the points again

System.out.println("p1 is (" + p1.x + ", " + p1.y + ")");

System.out.println("p2 is (" + p2.x + ", " + p2.y + ")");

}

}

class.ppt 13

Encapsulation

• Hiding the implementation details of an object from the clients of the object

• Abstraction – focusing on essential properties rather than inner details

• Encapsulation leads to abstraction

• Private fields

class.ppt 14

public class Point

{

private int x;

private int y;

public Point() //default constructor

{

x = 0;

y = 0;

}

public Point(int initialX, int initialY) // constructor

{

x = initialX;

y = initialY;

}

//returns the distance between this point and (0,0)

public double distanceFromOrigin()

{

return Math.sqrt(x*x + y*y);

}

//shifts this points location by the given amount

public void shift (int dx, int dy)

{

x += dx;

y += dy;

}

}

class.ppt 15

Accessing Class Members

• objects within a class definition can access public and private members

• objects out side of class definition can access only public members

class.ppt 16

Client

• Point point1 = new Point();

• point1.x = 7;

• point1.y = 3;

• Yields errors

• x had private access point in Point

• y has private access point in Point

class.ppt 17

Add new methods

public int getX()

{

return x;

}

public int getY()

{

return y;

}

public void setX(int newX)

{

x = newX;

}

public void getY(int newY)

{

y = newY;

}

class.ppt 18

Client codepublic class PointMain

{public static void main(String[] args)

{

Point p1 = new Point(7,4);

Point p2 = new Point(6,1);

//print each point and its distance from the origin

System.out.println("p1 is (" + p1.getX() + ", " + p1.getY() + ")");

System.out.println("distance from origin = " + p1.distanceFromOrigin());

System.out.println("p2 is (" + p2.getX() + ", " + p2.getY() + ")");

System.out.println("distance from origin = " + p2.distanceFromOrigin());

/ /shift

p1.setX(11);

p1.setY(6);

p2.shift(1,7);

//print the points again

System.out.println("p1 is (" + p1.getX() + ", " + p1.getY() + ")");

System.out.println("p2 is (" + p2.getX() + ", " + p2.getY() + ")");

}

}

class.ppt 19

Built-in Operations

• You cannot perform arithmetic operations on class objects– cannot use +, -, etc

• You cannot perform relational operations on objects– cannot use <, >, ==

• can use . to access public members• assignment is a shallow copy

class.ppt 20

class.ppt 21

List of terms used:

• Class = a structured type that is used to represent an ADT

• Class member = component of a class. Can be data or methods

• Class object (class instance) = a variable of a class type

• Client = software that declares and manipulates objects of a particular class.

• Data is generally private • Methods are generally declared public • Private class members can be accessed only by the

class member functions, not by client code.