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 Animals Stuff Dogs Stuff Animal Dog Dog extends (is-a) Animal

7 public class Animal { private int numLegs; public int getLegs() { return numLegs; } public void setLegs(int legs) { numLegs = legs; } public String noise() { return ?; } } 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!"; }

8 Inheritance Examples

9 Fig. 9.2Inheritance hierarchy for university CommunityMember s. CommunityMember EmployeeStudent StaffFaculty AdministratorTeacher Alumnus An Inheritance (Class) Hierarchy

10 Draw an Inheritance Hierarchy for these classes: Triangle Sphere TwoDimensionalShape Shape Pyramid Square ThreeDimensionalShape Cube Circle Sphere Shape TwoDimensionalShape CircleSquareTriangle ThreeDimensionalShape SphereCubePyramid

11 Try one more! WalkingBird Owl Ostrich Parrot Bird FlyingBird Chicken TalkingParrot Bird WalkingBird OstrichChicken FlyingBird Parrot TalkingParrot Owl

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

Bird call: ? color:? food:? movement:? WalkingBird call: ? color:? food:? movement:walk FlyingBird call: ? color:? food:? movement:fly Chicken call: cluck color: white food: bugs Ostrich call: neek-neek color: brown food: grass Parrot call: Squawk color:? food: fruit Owl call: hoo color: brown food:mice 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. Dont use protected instance variables! Fragile software can break if superclass changes

15 protected Members ClassA public m1() private m2() protected m3() ClassB ClassA.m1() ClassC ClassA.m1() ClassA.m3() 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 _________ 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 ____________. Private/Static/ Constructor Object extends cannot protected member