Inheritance in Java: Deriving New Classes and Supporting Software Reuse

1. Lecture 12 Inheritance

2. Inheritance • Objectives • Derive new classes from existing ones • How does inheritance supports software reuse • How to add and modify methods in child classes • Extend simple class derivations into well-designed class hierarchies • Discuss its use in java graphics frame

3. Introduction • Inheritance is the act of deriving a new class from an existing one • Derived class is a class created from an existing one • Eg: SavingsAccount is the derived class of Account class • The key idea here is the software reuse • Inheritance is the “is-a” relationship • Eg: Spinach “is-a” vegetable

4. Terminology • The original class is called • Parent class or • Superclass or • Base class • The derived class is called • Child class or • subclass

5. An Example - the base class book class Book { protected int pages = 1500; //---------------------------------------------------------------- // Prints a message concerning the pages of this book. //---------------------------------------------------------------- public void pageMessage () { System.out.println ("Number of pages: " + pages); } }

6. The derived class dictonary class Dictionary extends Book { private int definitions = 52500; //----------------------------------------------------------------- // Prints a message using both local and inherited values. //----------------------------------------------------------------- public void definitionMessage () { System.out.println ("Number of definitions: " + definitions); System.out.println ("Definitions per page: " + definitions/pages); } }

7. BOOK Dictionary

8. The Protected Modifier • Not all variables and methods are inherited in derivation • The visibility modifier (public, protected, private) decides which gets inherited • Protected variables retains the encapsulation properties but derived class will inherit it

9. The super reference • Constructors in base class are not inherited by the derived class, even if they have the public visibility • super is a general reference to a parent class • The parents constructor can be invoked using the super reference • It the super reference is included in the child’s constructor it must be the first line of the constructor

10. Example - book2 class Book2 { protected int pages; //---------------------------------------------------------------- // Sets up the book with the specified number of pages. //---------------------------------------------------------------- public Book2 (int pages) { this.pages = pages; } //---------------------------------------------------------------- // Prints a message concerning the pages of this book. //---------------------------------------------------------------- public void pageMessage () { System.out.println ("Number of pages: " + pages); } }

11. Example - Dictionary 2 class Dictionary2 extends Book2 { private int definitions; //----------------------------------------------------------------- // Sets up the dictionary with the specified number of pages // (maintained by the Book parent class) and defintions. //----------------------------------------------------------------- public Dictionary2 (int pages, int definitions) { super (pages); this.definitions = definitions; } //----------------------------------------------------------------- // Prints a message using both local and inherited values. //----------------------------------------------------------------- public void definitionMessage () { System.out.println ("Number of definitions: " + definitions); System.out.println ("Definitions per page: " + definitions/pages); } }

12. Multiple inheritance • Java’s approach to inheritance is called “single inheritance” • Languages like C++ allow multiple inheritance but Java does not • PickUpTruck “is-a” car and a truck • If parents classes have similar methods, which ones the child supports? • However the use of interface provides some of the abilities of multiple inheritance • A java class can implement many interfaces • It allows to interact with a particular class while inheriting most crucial information

13. Overriding methods • If a child class defines a method with the same name and signature, the child class overrides the parents method from the parent • Eg: Message class contains a method that instantiate two objects • One from Thought class • One from Advice class

14. Examples class Messages { //----------------------------------------------------------------- // Instatiates two objects a invokes the message method in each. //----------------------------------------------------------------- public static void main (String[] args) { Thought parked = new Thought(); Advice dates = new Advice(); parked.message (); dates.message (); // overridden } }

15. Class hierarchies • Multiple classes can be derived from a single parent. • Eg: animal class (parent), Rabbit, Cow, Cat (children) • Two children of the same parent are called siblings • Common features should be kept as high as possible in the class hierarchy • Inheritance mechanism is transitive • There are no single best hierarchy organization for all situations

16. The object class • In Java all classes are ultimately derived from the “object” class. • If the class definition doesn’t uses the extends clause, then it is automatically derived from the object class. • class Thing{ } • class Thing extends Object{ } • Are equivalent

17. Object class methods • boolean equals (Object obj); • Returns true if the objects are the same(aliases) • String toString(); • Returns a string representation of this object • Object clone(); • Creates and returns a copy of this object

18. Abstract Classes • An abstract class is a generic concept in a class hierarchy • Abstract class cannot be instantiated • Abstract class is similar to an interface • Any class that contains one or more abstract methods is declared to be abstract • Eg: public abstract class StaffMember{} • Eg: Vehicle (abstract), derive car, Van,….