Chapter 7 Generalization/Specialization and Inheritance

Chapter 7 - Generalization/Specialization and Inheritance 1

Chapter 7

Generalization/Specialization and Inheritance


Chapter 7 Topics• Creating generalization/specialization hierarchies

using the extends keyword to implement inheritance with problem domain classes

• Invoking superclass constructors when writing a subclass constructor

• Using the abstract and final keywords with Java superclasses

• Overriding superclass methods in subclasses• How polymorphism works with Java subclasses• The implications of protected and private access

in superclasses


Implementing the Boat Generalization/Specialization

Hierarchy• Boat Class (See Figure 7-2)

– Includes:• 4 attributes• Constructor invokes 4 accessor (setter) methods• 8 standard accessor methods

– 4 setters– 4 getters



Hierarchy• Testing the Boat Superclass

– TesterOne tester program (Figure 7-3)• Creates three boat instances• Retrieves information about each instance

– Sequence diagram (Figure 7-5)• Shows how TesterOne interacts with the Boat class

– Boat is ready to use as a superclass



Hierarchy• Using the Extends Keyword to Create the

Sailboat Subclass– Generalization/specialization hierarchy

• General superclass includes attributes and methods that are shared by specialized subclasses

– Instances of subclass inherit the attributes and methods of the superclass

– Include additional attributes and methods defined by subclass



Hierarchy• Using the Extends Keyword to Create

the Sailboat Subclass– Keywords:

• Use extends keyword to implement inheritance– public class Sailboat extends Boat

• Use super keyword to invoke superclass constructor

– super(aStateRegNo, aLength, aManufacturer, aYear);



Hierarchy• Using the Extends Keyword to Create

the Sailboat Subclass– To use inheritance:

• Need to know constructor and method signatures (APIs)

• Do not need:– Access to source code– Knowledge of how class is written



Hierarchy• Testing the Sailboat Subclass

– TesterTwoA tester program (Figure 7-8)• Creates 3 Sailboat instances and populates

them• Can use any methods defined in super or sub

class– Sequence diagram (Figure 7-10)

• Shows how TesterTwoA interacts with the Sailboat class



Hierarchy• Adding a Second Subclass – Powerboat

– Powerboat Class (Figure 7-11)• Second class can be extended from Boat without

effecting any other classes derived from Boat– TesterTwoB tester program (Figure 7-12)

• Creates instances of both subclass and populates them– Sequence diagram (Figure 7-14)

• Shows how TesterTwoB interacts with the Sailboat and Powerboat classes


Understanding Abstract and Final Classes

• Using Abstract Classes– Concrete class

• Class that can be instantiated– Abstract class

• Class not intended to be instantiated• Used only to extend into subclasses• Facilitates reuse• Keyword:

– abstract» Used in class header to declare an abstract class» e.g., public abstract class Boat


Understanding Abstract and Final Classes

• Using Final Classes– Final class

• Class not intended to be extended• Improves performance JVM does not have to search

for subclass methods that might override superclass methods

• Implements security by restricting class extension• Keyword:

– final» Used in class header to declare a final class» e.g. public final class Boat


Overriding a Superclass Method

• Method Overriding– Occurs when method in subclass is

invoked instead of method in superclass• Both methods have the same signature

– Allows subclass to modify the behavior of the superclass

• Compare overriding with overloading



• Adding & Overriding tellAboutSelf Method– Use same tellAboutSelf method signature in

subclass– Implement new functionality in overridden

method definition– Can invoke methods from super or sub class– Compare tellAboutSelf() in Figures 7-15, 7-

16



• Overriding & Invoking a Superclass Method– Two basic ways to override a superclass

method• Override completely – Saleboat 7-16

– Redefine what the method does• Override and append – Powerboat 7-19

– Override method– Call super to execute superclass method– Add additional functionality to overridden method



• Testing Two Method-Overriding Approaches– TesterThreeB tester program (Figure 7-20)

• Creates instances of both subclass, populates them, and demonstrates method overriding

– Sequence diagram• Shows how TesterThreeB interacts with the

Sailboat and Powerboat classes



• Overriding, Polymorphism, and Dynamic Binding– Polymorphism

• Objects of different classes can respond to the same message in their own way

• Simplifies processing:– Responsibility of responding appropriately is left

to the particular instance– Method overriding

• Form of polymorphism



• Overriding, Polymorphism, and Dynamic Binding– Dynamic Binding

• Resolves which method to invoke at runtime if overridden methods exist

• Provides flexibility when adding new subclasses to a system


Understanding Private Versus Protected Access

• Private access– No other object can directly read or

modify the value of an attribute• Must use accessor methods

– Also limits ability of subclass to directly access private variables of superclass• Must use accessor methods


Understanding Private Versus Protected Access

• Protected access– Subclass has direct access to protected

variables of superclass – pp. 236• Other classes in same package have direct access

as well– Can also use with methods– Use with care…

– Avoid use of public keyword with attributes• Violates encapsulation and information hiding

Chapter 7 Generalization/Specialization and Inheritance

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