2012 Pearson Education Inc All rights reserved Chapter

© 2012 Pearson Education, Inc. All rights reserved. Chapter 11: Inheritance Starting Out with Java: From Control Structures through Data Structures Second Edition by Tony Gaddis and Godfrey Muganda With formatting and other updates by Dr. L. Lilien

Inheritance (Chapter 11) – Topics • • • What Is Inheritance? Calling the Superclass Constructor Overriding Superclass Methods Protected Members Chains of Inheritance The Object Class Polymorphism Abstract Classes and Abstract Methods Interfaces © 2012 Pearson Education, Inc. All rights reserved. 11 -2

11. 1. What is Inheritance? Generalization vs. Specialization • Real-life objects are typically specialized versions of other more general objects • NEXT PAGE - Example : – Insect -- a very general type of creature (with numerous characteristics) – Grasshoppers and bumblebees are insects • They share the general characteristics of an insect. • However, they have special characteristics of their own: – grasshoppers -- a jumping ability – bumblebees -- a stinger • Grasshoppers and bumblebees are specialized versions of an insect © 2012 Pearson Education, Inc. All rights reserved. 11 -3

Inheritance Insect Contains attributes and methods shared by all insects. (a superclass) Bumble. Bee Grasshopper (an inherited class) Contains attributes and methods specific to a Bumble Bee (a stinger, …) © 2012 Pearson Education, Inc. All rights reserved. Contains attributes and methods specific to a Grasshopper (jumping ability, . . . ) 11 -4

The “is a” Relationship • The “is a” relationship — the relationship between a superclass and an inherited class – A grasshopper “is an” insect – A poodle “is a” dog – A car “is a” vehicle • Characteristics of a specialized object include: – Those of its general object, plus – Those that make it special • In object-oriented programming, inheritance used to create an “is a” relationship among classes © 2012 Pearson Education, Inc. All rights reserved. 11 -5

The “is a” Relationship • Inheritance involves: – a superclass = a general class = a “parent” class – a subclass = a specialized class = a “child” class • A subclass extends (is based on) its superclass. – a superclass = a base class – a subclass = a derived class © 2012 Pearson Education, Inc. All rights reserved. 11 -6

Inheritance • The subclass inherits fields and methods from the superclass – without any of them being rewritten • In addition, the subclass may have new fields and methods • The Java keyword extends is used on the class header to define the subclass. public class Final. Exam extends Graded. Activity © 2012 Pearson Education, Inc. All rights reserved. 11 -7

The Graded. Activity Example Graded. Activity - score : double + set. Score(s : double) : void + get. Score() : double + get. Grade() : char NOTE: “-” = private attribute/method “+” = public attribute/method Contains attributes and methods shared by all graded activities. Inherits all non-private attributes and methods from Graded. Activity. Contains attributes and methods specific to Final. Exam. Fina. Exam - num. Questions : int - points. Each : double - num. Missed : int + Final. Exam(questions : int, missed : int) + get. Points. Each() : double + get. Num. Missed() : int © 2012 Pearson Education, Inc. All rights reserved. • Example: – – Graded. Activity. java, Grade. Demo. java, Final. Exam. Demo. java 11 -8

Inheritance, Fields and Methods • Private members of the superclass: – are not inherited by the subclass – exist in memory when the object of the subclass is created – subclass may accessed them only via public methods of the superclass • Public members of the superclass: – are inherited by the subclass – may be directly accessed from the subclass © 2012 Pearson Education, Inc. All rights reserved. 11 -9

Inheritance, Fields and Methods • When a subclass instance is created, the non-private methods of the superclass are available through the subclass object. Final. Exam exam = new Final. Exam(); exam. set. Score(85. 0); // set. Score is non-private // in superclass System. out. println("Score = " + exam. get. Score()); • Non-private methods and fields of the superclass are available in the subclass. set. Score(numeric. Score); © 2012 Pearson Education, Inc. All rights reserved. 11 -10

Inheritance and Constructors • Constructors are not inherited • When a subclass is instantiated, the superclass default constructor (provided by Java) or no-argument constructor (provided by programmer) is executed first (before the subclass’s own constructor) – Example: • Super. Class 1. java • Sub. Class 1. java • Constructor. Demo 1. java © 2012 Pearson Education, Inc. All rights reserved. 11 -11

11. 2. Calling the Superclass’s Constructor • The super keyword refers to an object’s superclass • The superclass constructor can be explicitly called from the subclass by using the super keyword. – Examples: • Super. Class 2. java, Sub. Class 2. java, Constructor. Demo 2. java ++READ++ • Rectangle. java, Cube. Demo. java © 2012 Pearson Education, Inc. All rights reserved. 11 -12

Calling The Superclass Constructor • A superclass may have parameterized constructors only • That is, no programmer-provided no-argument constructor – In this case, Java does not provide a default constructor automatically • RECALL: Java provides the default constructor only when a class has no programmer-provided constructors (parametrized or not) © 2014 Leszek T. Lilien © 2012 Pearson Education, Inc. All rights reserved. 11 -13

Calling The Superclass Constructor – cont. • If a superclass has (only) parameterized constructors, (does not have deafult constructor or no-argument constructor), – subclasses must provide constructors AND – each subclass constructor must call one of constructors of the superclass © 2014 Leszek T. Lilien © 2012 Pearson Education, Inc. All rights reserved. 11 -14

Calling The Superclass Constructor – cont. • If >1 superclass constructors, which one is called when subclass object created? – A subclass can explicitly call a selected superclass constructor with the super keyword • This call must be the first Java statement in the subclass constructor © 2014 Leszek T. Lilien © 2012 Pearson Education, Inc. All rights reserved. 11 -15

11. 3. Overriding Superclass Methods • Recall that a method signature consists of: – the method’s name – the data types of method’s parameters • in the order that they appear • Example: – Graded. Activity. java, includes the method: public void repl(int num, String id) – Its signature: repl(int, String) © 2014 Leszek T. Lilien © 2012 Pearson Education, Inc. All rights reserved. 11 -16

Overriding Superclass Methods • A subclass may have a method with the same signature as a superclass method. – In this case, the subclass method overrides the superclass method • Example of method overriding: – Graded. Activity. java, Curved. Activity. Demo. java © 2012 Pearson Education, Inc. All rights reserved. 11 -17

Overriding Superclass Methods Graded. Activity - score : double + set. Score(s : double) : void + get. Score() : double + get. Grade() : char Curved. Activity - raw. Score : double - percentage : double This setscore method is a more specialized version of the set. Score method in the superclass. + Curved. Activity(percent : double) + set. Score(s : double) : void + get. Raw. Score() : double + get. Percentage() : double © 2012 Pearson Education, Inc. All rights reserved. 11 -18

Overriding Superclass Methods • A subclass method that overrides a superclass method must have the same signature as the superclass method being overriden – An object of the subclass invokes the subclass’s version of the method, not the superclass’s • Unless super. method_name used (next slide) © 2012 Pearson Education, Inc. All rights reserved. 11 -19

Overriding Superclass Methods • An subclass method can call the overridden superclass method via the super keyword. super. set. Score(raw. Score * percentage); © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -20

Overriding vs. Overloading • Overloading a method vs. overriding a method – Overloading is when a method has the same name as one or more other methods, but with a different signature – When a method overrides another method, however, they both have the same signature • Overloading and overriding can take place in an inheritance relationship – Overriding - only in an inheritance relationship • Example: – Super. Class 3. java, – Sub. Class 3. java, – Show. Value. Demo. java © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -21

Preventing a Method from Being Overridden • The final modifier will prevent the overriding of a superclass method in a subclass. public final void message() • If a subclass attempts to override a final method, the compiler generates an error. – Ensures that a particular superclass method is used by subclasses rather than an overriding version of it. © 2012 Pearson Education, Inc. All rights reserved. 11 -22

11. 4. Protected Members • Java provides a third access specification, protected – In addition to public and private – A protected member’s access is somewhere between private and public. • In UML diagrams, indicated with # • Protected members of class: – may be accessed by methods in a subclass, and – by methods in the same package as the class • Example: – Graded. Activity 2. java – Final. Exam 2. java – Protected. Demo. java © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -23

Protected Members • Avoid the protected access specificaton – Using protected instead of private makes some tasks easier – However, any class that is derived from the class, or is in the same package, has unrestricted access to the protected member • It is always better to make all fields private and then provide public methods for accessing those fields • If no access specifier for a class member is provided, the class member is given package access by default – Any method in the same package may access the member © 2012 Pearson Education, Inc. All rights reserved. 11 -24

Access Specifiers Access Modifier Accessible to a subclass inside the same package? Accessible to all other classes inside the same package? default (no modifier) Yes Public Yes Protected Yes Private No No Accessible to a subclass outside the package? Access Modifier Accessible to all other classes outside the package? default (no modifier) No No Public Yes Protected Yes No Private No No © 2012 Pearson Education, Inc. All rights reserved. 11 -25

++READ++ 11. 5. Chains of Inheritance • A superclass can be a subclass of another class. Object Example: Graded. Activity. java Pass. Fail. Exam. Demo. java Graded. Activity Pass. Fail. Exam © 2012 Pearson Education, Inc. All rights reserved. 11 -26

++READ++ Chains of Inheritance • Classes often are depicted graphically in a class hierarchy. • A class hierarchy shows the inheritance relationships between classes. Graded. Activity Final. Exam Pass. Fail. Activity Pass. Fail. Exam © 2012 Pearson Education, Inc. All rights reserved. 11 -27

11. 6. The Object Class • All Java classes are directly or indirectly derived from a class named Object – Object is in the java. lang package – Any class that does not specify the extends keyword is automatically derived from the Object class • Example: public class myclass { // This class is implicitly derived from Object. } The above class is equivalent to: public class My. Class extends Object { // This class is explicitly derived from Object. } © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -28

The Object Class • Every class inherits the Object class’s members. – Because every class is directly or indirectly derived from the Object class • Every class inherits Object class’s to. String and equals methods – to. String returns a string containing the object’s class name and a hash of its memory address – equals checks if 2 object references denote the same object • equals accepts the address of an object as its argument and returns true if it is the same as the calling object’s address • Example: Object. Methods. java © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -29

11. 7. Polymorphism Graded. Activity Final. Exam • Declare a reference variable for a superclass Graded. Activity exam; Pass. Fail. Activity Pass. Fail. Exam // It can (obviously) reference a superclass object exam = new Graded. Activity(); • Final. Exam is a subclass of Graded. Activity – So, any Final. Exam subclass object is-a Graded. Activity superclass object – Therefore, a superclass Graded. Activity variable may be used to reference a subclass Final. Exam object Graded. Activity exam = new Final. Exam(50, 7); © 2014 Lilien 2012 Leszek Pearson. T. Education, Inc. All rights reserved. 11 -30

Polymorphism • REPEATING: a superclass Graded. Activity reference variable may be used to reference a subclass Final. Exam object Graded. Activity exam = new Final. Exam(50, 7); • This statement creates a Final. Exam object and stores the object’s address in the exam variable (of the superclass type) – This is an example of polymorphism • polymorphism = the ability to take many forms • In Java, a reference variable is polymorphic because it can reference objects of types different from its own, as long as those types are subclasses of its type © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -31

Polymorphism Graded. Activity Final. Exam • Other legal polymorphic references: Pass. Fail. Activity Pass. Fail. Exam Graded. Activity exam 1 = new Final. Exam(50, 7); Graded. Activity exam 2 = new Pass. Fail. Activity(70); Graded. Activity exam 3 = new Pass. Fail. Exam(100, 10, 70); • The Graded. Activity class has three methods: set. Score, get. Score, and get. Grade. • A Graded. Activity variable can be used to call only its three methods (not any method of any of its subclass) Graded. Activity exam = new Pass. Fail. Exam(100, 10, 70); System. out. println(exam. get. Score()); // This works. System. out. println(exam. get. Grade()); // This works. System. out. println(exam. get. Points. Each()); // ERROR! A PFE method © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -32

Polymorphism and Dynamic Binding Graded. Activity Final. Exam Pass. Fail. Activity Pass. Fail. Exam • Suppose that a subclass has overridden a method in the superclass: – Then, if the variable makes a call to that method the subclass’s version of the method will be run Graded. Activity exam = new Pass. Fail. Activity(60); exam. set. Score(70); // set. Score in GA not in PFA System. out. println(exam. get. Grade()); // get. Grade in GA and in PFA; JVM calls PFA’s get. Grade • Java performs dynamic binding or late binding when a variable contains a polymorphic reference – The Java Virtual Machine (JVM) determines at runtime which method to call • Depending on the type of object that the variable references © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -33

Polymorphism • The object’s type (not the reference type) determines which method is called Graded. Activity exam = new Pass. Fail. Activity(60); // exam is an object of type PFA System. out. println(exam. get. Grade()); // get. Grade in both GA and in PFA // JVM calls PFA’s get. Grade since the object exam // is of type PFA (although the exam reference // variable is of superclass type GA) • Example: – Polymorphic. java • You cannot assign a superclass object to a subclass reference variable © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -34

The instanceof operator • See textbook, p. 701 © 2014 Lilien 2012 Leszek Pearson. T. Education, Inc. All rights reserved. 11 -35

11. 8. Abstract Classes and Abstract Methods Abstract Classes • An abstract class cannot be instantiated – But still serves as a superclass for other classes – Represents the generic or abstract form of all the classes that are derived from it • Indicated with the abstract keyword in the class definition public abstract class Class. Name © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -36

Abstract Methods • An abstract class may include abstract methods • An abstract method has no body (only a header) • It must be overridden in a subclass Access. Specifier abstract Return. Type Method. Name(Params); e. g. : public abstract void set. Value(int value); • Example: – Student. java, Comp. Sci. Student. Demo. java © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -37

Abstract Methods • Notice that the key word abstract appears in the header, and that the header ends with a semicolon. • Any class that contains an abstract method is automatically abstract • If a subclass fails to override an abstract method, a compiler error will result • Abstract methods are used to ensure that a subclass implements the method © 2012 Pearson Education, Inc. All rights reserved. 11 -38

11. 9. Interfaces • An interface is similar to an abstract class that has all abstract methods (not just some abstract methods , as an abstract class) – The keyword interface indicates an interface public interface Interface. Name { (Method headers. . . ) } – An interface cannot be instantiated – All of the methods of an interface must be written elsewhere • They have no bodies, only headers – All methods in an interface are public by default • The purpose of an interface: to specify behavior for other classes © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -39

Interfaces • A class can implement one or more interfaces. • Example: A class implementing an interface - the implements keyword public class Final. Exam 3 extends Graded. Activity implements Relatable • Example: – – Graded. Activity. java Relatable. java Final. Exam 3. java Interface. Demo. java © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -40

Fields in Interfaces • An interface can contain field declarations: – All fields in an interface are final and static • Because they are final, you must initialize them public interface Doable { int FIELD 1 = 1, FIELD 2 = 2; (Method headers. . . ) } – FIELD 1 and FIELD 2 are final static int variables. – Any class that implements this interface has access to these variables. © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -41

Implementing Multiple Interfaces • A class can implement multiple interfaces – In contrast, a class can be derived from only one superclass public class My. Class implements Interface 1, Interface 2, Interface 3 • When a class implements multiple interfaces, it must provide the methods specified by all of them © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -42

Interfaces in UML (see p. 715) Graded. Activity Final. Exam 3 © 2012 Pearson Education, Inc. All rights reserved. 1) A dashed arrow indicates implementation of an interface (known as a realization—the class realizes the interface). 2) The <<interface>> tag precedes the interface name. 3) The interface name in italics. <<interface>> Relatable Updated by L. Lilien 11 -43

Polymorphism with Interfaces • Java interface reference variable – It can reference any object that implements that interface, regardless of its class type • This is interface polymorphism – Example: Assume that classes Compact. Disc and Dvd. Movie realize the Retail. Item interface Retail. Item item 1 = new Compact. Disc( “Songs from the Hart”, “Billy Nelson”, 18. 95); Retail. Item item 2 = new Dvd. Movie( “Planet X”, 102, 22. 95); – In the example code, two Retail. Item reference variables, item 1 and item 2, are declared. – The item 1 variable references a Compact. Disc object and the item 2 variable references a Dvd. Movie object © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -44

Polymorphism with Interfaces • When a class implements an interface, an inheritance relationship known as interface inheritance is established – For example: • any Compact. Disc object (incl. item 1) is a Retail. Item, and • any Dvd. Movie object (incl. item 2) is a Retail. Item • Example: – – Retail. Item. java Compact. Disc. java Dvd. Movie. java Polymorphic. Interface. Demo. java © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -45

Polymorphism with Interfaces • An interface reference variable can refer to any class that implements (realizes) that interface • You cannot create an instance of an interface Retail. Item item = new Retail. Item(); // ERROR! • When an interface variable references an object: – Only the methods declared in the interface are available – Explicit type casting is required to access the other methods of an object referenced by an interface reference See example on p. 720 © 2012 Pearson Education, Inc. All rights reserved. Updated by L. Lilien 11 -46

The End © 2012 Pearson Education, Inc. All rights reserved. 11 -47