Object Oriented Programming Concepts

1. 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

2. 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

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 class.ppt

4. 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

5. Declaring objects • Done in client • Point p1; // no space allocated • Example: • Point point1 = new Point(); //allocates spacePoint 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. Point point1; x 5 y 30 Point point2; x 17 y 58 Class Objects or Class Instances class.ppt

7. Client • Point point1 = new Point(); • point1.x = 7; • point1.y = 3; • point1.shift(2,-1); • System.out.println(p1.distanceFromOrigin()); class.ppt

8. 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

9. 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

10. Constructors public Point() { x = 0; y = 0; } public Point(int initialX, int initialY) { x = initialX; y = initialY; } class.ppt

11. 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

12. Client code • Point p1 = new Point(); //calls default • Point p2 = new Point(6,1); class.ppt

13. Client code public 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

14. 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

15. 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

16. 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

17. 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

18. Add new methods public int getX() { return x; } public int getY() { return y; } public int setX(int newX) { x = newX; } public int getY(int newY) { y = newY; } class.ppt

19. Client code public 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

20. 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

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. class.ppt