Java Programming, Third Edition

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Java Programming, Third Edition

• Chapter Twelve
• Advanced Inheritance Concepts

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Objectives

• Create and use abstract classes
• Use dynamic method binding
• Create arrays of subclass objects
• Use the Object class and its methods

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Objectives (continued)

• Use inheritance to achieve good software design
• Create and use interfaces
• Create and use packages

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Creating and Using Abstract Classes

• Superclass contains features shared by its
  subclasses
• Abstract class is a superclass that is not
  instantiated
• Abstract class is a template for subclasses
• Keyword abstract used to create abstract class
• Abstract classes have abstract and non-abstract
  methods

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Creating and Using Abstract Classes (continued)

• Abstract methods are empty
• Two ways to create abstract method
• Use abstract keyword
• Create empty method within an abstract class
• Concrete child must override inherited abstract
  methods
• Example abstract class named Animal
• String attribute called nameOfAnimal
• Non-abstract setAnimalName( ) and getAnimalName(
  )
• Abstract method named speak( )

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Creating and Using Abstract Classes (continued)

• Create three classes extending Animal
• Dog, Cow, and Snake
• Override speak( ) within each child class

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Creating and Using Abstract Classes (continued)

• Demonstrate three methods in UseAnimals driver
• Instantiate Dog, Cow, Snake objects
• Utilize non-abstract setter and getter from
  parent
• Call specific versions of speak( )
• Abstract Animal class supports polymorphism
• Polymorphism one method name referring to many
  implementations

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Using Dynamic Method Binding

• Each object of a subclass is a superclass object
• You may convert subclass objects to superclass
  objects
• You may create reference to abstract superclass
  object
• Reference variable points to a memory address
• Reference is declared with name only, like
  primitive
• Reference to abstract superclass may store child
  class object
• Example AnimalReference
• Animal holds Cow object and then Dog object

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Using Dynamic Method Binding (continued)

• Call to speak( ) based on reference content
• A form of polymorphism
• Binding class method to object occurs at run-time
• Dynamic method binding
• Technique behind virtual method calls
• Correct method bound to object based on current
  (dynamic) context

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Creating Arrays of Subclass Objects

• An Array of abstract class references is legal
• Stores different child objects with same ancestry
• Example AnimalArray class
• Array declaration Animal ref new Animal3
• Statement does not instantiate Animal objects
• Statement reserves room for Animal object
  references
• All array operations valid

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Using the Object Class and its Methods

• Object class is ancestor of every class coded in
  Java
• Your class extends Object or a descendant of
  Object
• Object class is defined in the java.lang package
• java.lang automatically imported
• You may use or override Object class methods

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The toString( ) Method

• Object class toString( ) method
• Converts Object into a String
• String contains information about Object data
• String value returned by toString( )
• Class name of which object is an instance
• _at_ symbol
• Hexadecimal address (memory location) of object
• Example DogToString application
• Call Object class toString( ) against Dog
  instance
• Output Dog_at_1ac04e8

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The toString( ) Method (continued)

• Object class toString( ) method may be overridden
• User-defined toString( ) returns useful data
• toString( ) helpful in debugging applications
• Example BankAccountClass
• Explicit toString( ) method returns attributes
• toString( ) called in TestBankAccount
• Bug inferred from returned data
• balance attribute improperly initialized

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The equals( ) Method

• Object class equals( ) takes single argument
• Argument identical in type to calling object
• SomeObject.equals(anOtherObjectOfTheSameType)
• Returns true if objects are equal
• Object class equals( ) view of equality
• Both objects have the same memory addresses
• One object is a reference to another
• Example CompareAccounts application
• Two BankAccount objects initialized with same
  values
• Object equals( ) returns false (addresses not the
  same)

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The equals( ) Method (continued)

• Object class equals( ) may be overridden
• User expects equals( ) to compare attributes
• Example BankAccount equals( ) method
• Compares attributes of two BankAccount objects
• Returns true if attribute values identical
• Called against two objects in earlier example

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Using Inheritance to Achieve Good Software Design

• Features of good software design
• Correctness, reliability, robustness,
  reusability, efficiency, ease of maintenance
• Inheritance is part of good software design
• Development time reduced (build on ancestors)
• Debugging time reduced (ancestors tested)
• Learning time reduced (superclass basis known)
• Integrity maintained (superclass separately
  compiled)

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Creating and Using Interfaces

• Multiple inheritance class derives from more
  than one superclass
• Java does not allow multiple inheritance
• Rationale for banning inheritance
• Confused inheritance lines
• Possible name conflict
• Ambiguity involved in binding super( )
• Interface is an alternative to multiple
  inheritance

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Creating and Using Interfaces (continued)

• Interface describes what a class does, not how it
  does it
• Features specific to an interface
• All methods are implicitly abstract and final
• All data is implicitly public, static, and final
• Interface methods have full headers, but empty
  bodies
• Create an interface using implements keyword
• User subclass must define each interface method
• Ex WorkingDog extends Dog implements Worker

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Creating and Using Interfaces (continued)

• WorkingDog tested in DemoWorkingDogs
• Two WorkingDog objects use the following methods
• setAnimalName(), getAnimalName() inherited from
  Animal
• speak( ) inherited from the Dog (which extends
  Animal)
• setHoursOfTraining( ), getHoursOfTraining( ) in
  WorkingDog
• work( ) implemented through Worker interface

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Creating Interfaces to Store Related Constants

• Interfaces contain data that is public, static,
  final
• Purpose to using constants in interface
• Provide data that can be reused without
  re-declaration
• Example PizzaConstants
• Provides a number of constants for a pizzeria
• Class implementing interface can use permanent
  values

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Creating and Using Packages

• Package named collection of classes
• Examples java.lang, java.util, javax.swing
• You may create your own packages
• Conveniently store related classes
• May be imported by other programmers
• Mechanics of creating a package
• Include package statement at beginning of class
  file
• Statement indicates folder to place class
• Example package com.course.animals

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Creating and Using Packages (continued)

• Mechanics of using a package
• Use import statement at top of class file
• Example import com.course.animals.
• Wild card () provides access to all packaged
  classes
• Package names should be unique
• Naming convention
• Internet domain name in reverse order
• Ex If domain name is course.com, use com.course

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Summary

• Each object of subclass is a superclass object
• Abstract classes may be extended, not
  instantiated
• Abstract methods have full headers, empty bodies
• User subclasses must define abstract methods
• You can convert subclass objects to superclass
  object

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Summary (continued)

• Abstract classes may be declared as reference
• You may store concrete subclass in abstract
  reference
• Dynamic method binding select method at run-time
• Object class is ancestor of every Java class
• Object class methods may be used or overridden

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Summary (continued)

• Interface methods are implicitly public,
  abstract, final
• Interface data is implicitly public, static,
  final
• Keyword implements provides access to interface
• Interface methods must be defined by user class
• Use packages to conveniently group classes