COP 3503 FALL 2012 Shayan Javed Lecture 4

1. COP 3503 FALL 2012ShayanJavedLecture 4 Programming Fundamentals using Java

2. Inheritance • Derive new classes (subclass) from existing ones (superclass). • Only the Object class (java.lang) has no superclass • Every class = subclass of Object.

3. Inheritance • Code reuse – methods and properties. • Use classes to model objects of the same type • Define general class • Then define specialized classes.

4. Inheritance • Can only inherit from one class. • Can inherit from multiple classes in other languages (C++)

5. Example • Let’s say you create the following classes: • Circle • Rectangle • Triangle • etc. • Share certain properties, but also specialized properties

6. Example public class GeometricObject { private String Color; private String name; private float area; // constructors... publicGeometricObject(String color, String name) { … } // get/set methods, etc. }

7. Example public class Circle extendsGeometricObject { private double radius; // specific property public Circle(double radius, String color, String name) { this.radius = radius; this.color = color; this.name = name; } // get/set methods, etc. publicdoublegetArea() { return radius * radius * Math.PI; } }

8. Example • An error in the constructor! public Circle(double radius, String color, String name) { this.radius = radius; this.color = color; this.name = name; } • Why can’t we use “this.color” and “this.name”?

9. Example • They are private properties. • NOT accessible by subclasses. • Should use setColor() instead. • public methods/properties inherited.

10. Example public class Rectangle extendsGeometricObject { private double width, height; // specific properties public Rectangle(double w, double h, String color, String name) { this.height = h; this.width = w; setColor(color); setName(name); } // get/set methods, etc. publicdoublegetArea() { return width * height; } }

11. The super keyword • Are constructors inherited? • Yes. super used for: • calling a super class constructor • calling a super class method

12. The super keyword • Constructors: • super() calls the default constructor • super(arguments) calls the constructors according to the arguments

13. The super keyword • Constructors: public Circle(double radius, String color, String name) { super(color, name); this.radius = radius; }

14. The super keyword • Constructors: public Circle(double radius, String color, String name) { super(color, name); this.radius = radius; } publicGeometricObject(String color, String name) { this.color = color; this.name = name; }

15. The super keyword What if we don’t add the super(..) constructor call?

16. The super keyword public Circle(double radius, String color, String name) { this.radius = radius; }

17. The super keyword public Circle(double radius, String color, String name) { this.radius = radius; } Is the same as: public Circle(double radius, String color, String name) { super(); this.radius = radius; }

18. The super keyword Called Constructor Chaining

19. The super keyword • Calling the super class methods • super.method(parameters)... • Optional

20. Using get/set methods to access properties of the superclass is cumbersome…

21. The protected keyword • Often want subclasses to directly access properties/methods • Use protected instead of private/public • Subclasses can now directly access

22. Private public class GeometricObject { private String Color; private String name; private float area; // constructors... // get/set methods, etc. }

23. The protected keyword public class GeometricObject { protected String Color; protected String name; protected float area; // constructors... // get/set methods, etc. }

24. The protected keyword public class Circle extendsGeometricObject { private double radius; // specific property public Circle(double radius, String color, String name) { this.radius = radius; this.color = color; this.name = name; } } Now works!

25. Polymorphism • Important aspect of OOP

26. Polymorphism • Important aspect of OOP • “capability of an action or method to do different things based on the object that it is acting upon. ”

27. Polymorphism • Important aspect of OOP • “capability of an action or method to do different things based on the object that it is acting upon. ” • “characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.”

28. Polymorphism • Overriding Methods • Overloaded methods • Dynamic (late) method binding

29. Overriding Methods • Sometimes need to overwrite methods written in the superclass • Re-define the method in the subclass.

30. Overriding Methods • The Object class and toString() • toString() = String output when you print the object

31. Overriding Methods • The Object class and toString() • toString() = String output when you print the object • Object class has default toString() method

32. The toString() method publicStringtoString() { String str; ... return str; }

33. The toString() method Circle circle1 = newCircle(“Circle1”, “Red”, 3.5); System.out.println(circle1); Output: ?

34. The toString() method Circle circle1 = newCircle(“Circle1”, “Red”, 3.5); System.out.println(circle1); Output: Circle@19821f

35. The toString() method • So override the method publicStringtoString() { String str = “”; str += “Circle: ” + getName() + “,”; str += “Color: ” + getColor() + “,”; str += “Radius: ” + getRadius(); return str; }

36. The toString() method Circle circle1 = newCircle(“Circle1”, “Red”, 3.5); System.out.println(circle1); Output: Circle: Circle1,Color: Red,Radius: 3.5

37. The equals() method • The Object class and equals() • equals (Object obj) = returns whether the object passed in and this one are the same

38. The equals() method • Default implementation: publicbooleanequals(Object obj) { return (this == obj); } Compares references

39. The equals() method Circle circle1 = newCircle(3.5); Circle circle2 = newCircle(3.5);

40. The equals() method Circle circle1 = newCircle(3.5); Circle circle2 = newCircle(3.5); circle1.equals(circle2);// ?

41. The equals() method Circle circle1 = newCircle(3.5); Circle circle2 = newCircle(3.5); circle1.equals(circle2);// returns false!

42. The equals() method Circle circle1 = newCircle(3.5); Circle circle2 = newCircle(3.5); circle1.equals(circle2);// returns false! Circle circle3 = circle1; circle1.equals(circle3); // ?

43. The equals() method Circle circle1 = newCircle(3.5); Circle circle2 = newCircle(3.5); circle1.equals(circle2);// returns false! Circle circle3 = circle1; circle1.equals(circle3); // returns true!

44. The equals() method • Override it publicbooleanequals(Object obj) { if (objinstanceof Circle) return this.radius == ((Circle)obj).getRadius(); else return false; }

45. The instanceofoperator • Test if object is of a specified type if (object instanceof Class)

46. The instanceofoperator • Test if object is of a specified type if (object instanceof Class) • Example: Circle circle1 = new Circle(3.4); if (circle1 instanceofCircle) { … }

47. The instanceofoperator • Testing with the SuperClass: if (subclassObjinstanceofSuperClass) { … } ?

48. The instanceofoperator • Testing with the SuperClass: if (Circle instanceofGeometricObject) { … } ?

49. The instanceofoperator • Testing with the SuperClass: if (subclassObjinstanceofSuperClass) { … } returns true!

50. The instanceofoperator • Example: Circle circle1 = new Circle(3.4); if (circle1 instanceofGeometricObject) { … } returns true!