What is Inheritance ?

1. What is Inheritance? • A form of software reuse • Create new class from existing class • Absorb existing class’ data and behaviors • Enhance with new or modified capabilities • Used to eliminate redundant code • Example • Circle class inherits from Shape class • Circle extends Shape

2. Subclass and Superclass • Subclass extends superclass • Subclass • Also called child class or derived class • More specialized group of objects • Inherits data and behaviors from superclass • Can add or modify behaviors (methods) • Superclass • Also called parent class or base class • Typically represents larger group of objects • Supplies data and behaviors to subclass • Direct and indirect • Single and multiple inheritance

3. The Object class • Top of the Java class hierarchy • Located in package java.lang • Class from which every other Java class inherits • A class implicitly extends Object if no other class is specified • .toString(), .clone(), .equals() equals() compares addresses, not the contents of objects

4. “is-a” vs. “has-a” relationships • “is-a” • Represents inheritance • subclass object is an example of the superclass object • Example: a Car is a Vehicle • Car is subclass; Vehicle is superclass • Keywords: extends, implements • “has-a” • Represents composition • Object contains one or more objects of other classes as members • Example: Car has a Steering Wheel

5. Composition • Dog “has-a” owner, mother, father, leash… public class Dog{ private String name; private int age; private Person owner; private Dog mother, father; private Leash dogLeash;}

6. Inheritance • Dog extends (“is-a”) Animal Animal Animal’s Stuff Dog Animal’s Stuff Dog’s Stuff

7. public class Dog extends Animal { public static void main (String[] args) { Dog d = new Dog(); d.setLegs(4); System.out.println ("A dog has " + d.getLegs() + " legs"); System.out.println ("A dog says " + d.noise()); } public String noise() { return "WOOF!"; } } public class Animal{ private int numLegs; public int getLegs() { return numLegs; } public void setLegs(int legs) { numLegs = legs; } public String noise() { return “?”; }}

8. Inheritance Examples

9. CommunityMember Employee Student Alumnus Faculty Staff Administrator Teacher An Inheritance (Class) Hierarchy Fig. 9.2 Inheritance hierarchy for university CommunityMembers.

10. Draw an Inheritance Hierarchy for these classes: TriangleSphere TwoDimensionalShape ShapePyramidSquareThreeDimensionalShapeCubeCircleSphere

11. Try one more! WalkingBirdOwlOstrichParrotBirdFlyingBirdChickenTalkingParrot

12. Strategy • Design classes for objects • Identify characteristics classes have in common • Abstraction: focus on commonalities among objects in a system • Design superclasses to store common characteristics

13. Bird call: ?color:?food:?movement:? FlyingBird call: ?color:?food:?movement:fly WalkingBird call: ?color:?food:?movement:walk Parrot call: Squawkcolor:?food: fruit Owl call: hoocolor: brownfood:mice Chicken call: cluckcolor: whitefood: bugs Ostrich call: neek-neekcolor: brownfood: grass TalkingParrot. . .

14. protected Members • Variables or methods available only to derived classes • Intermediate level of protection between public and private • Accessible to • superclass members • subclass members • class members in the same package • Use super. to access a superclass method that has been overridden by a subclass method. • Don’t use protected instance variables! • “Fragile” software can “break” if superclass changes

15. protected Members ClassC ClassA.m1()ClassA.m3() ClassA public m1()private m2()protected m3() ClassB ClassA.m1() is-a has-a

16. Constructors in Subclasses • Constructors are not inherited! • Chain of constructor calls • subclass constructor invokes superclass constructor • Implicitly or explicitly • To call explicitly, use super() • Superclass constructor call must be first statement in subclass constructor • Object constructor is always fired last • Example: Student extends Person

17. Software Engineering with Inheritance • Customizing existing software • Inherit from existing classes • Include additional members • Redefine superclass members • No direct access to superclass source code • Independent software vendors (ISVs) • Develop proprietary code for sale/license • Users derive new classes • Without accessing ISV proprietary source code

18. Quiz Private/Static/Constructor • _________ methods are not inherited. • The _____ class is at the top of the Java hierarchy. • The _____ keyword is used in Java to represent inheritance. • A Java class _____ inherit from more than one class. • A variable or method that is available only to derived classes is called a____________. Object extends cannot protected member