Chapter 7 Generalization/Specialization and Inheritance

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Chapter 7Generalization/Specialization and
Inheritance
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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

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Implementing the Boat Generalization/Specializatio
n Hierarchy

• Boat Class (See Figure 7-2)
• Includes
• 4 attributes
• Constructor ? invokes 4 accessor (setter) methods
• 8 standard accessor methods
• 4 setters
• 4 getters

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Implementing the Boat Generalization/Specializatio
n 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

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Implementing the Boat Generalization/Specializatio
n 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

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Implementing the Boat Generalization/Specializatio
n 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)

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Implementing the Boat Generalization/Specializatio
n 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

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Implementing the Boat Generalization/Specializatio
n 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

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Implementing the Boat Generalization/Specializatio
n 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

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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

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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

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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

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Overriding a Superclass Method

• 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

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Overriding a Superclass Method

• 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

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Overriding a Superclass 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

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Overriding a Superclass Method

• 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

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Overriding a Superclass Method

• 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

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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

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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