Chapter 11 Inheritance

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Chapter 11 -- Inheritance

• Jim Burns

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The Concept of Inheritance

• Since day one, you have been creating classes and
  instantiating objects that are members of those
  classes.
• Programmers use a graphical language to describe
  classes and object-oriented processesthis
  Unified Modeling Language consists of many types
  of diagrams

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Class diagram

• Is a visual tool that provides you with an
  overview of a class
• It is a rectangle divided into three sections
• the top section contains the name of the class
• The middle section contains the names and data
  types of the attributes
• The bottom section contains the methods

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The Employee class diagram
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Another employee class

• Suppose that a new employee called serviceRep is
  hired and that, in addition to employee number
  and salary, he needs a territory.
• Rather than creating a whole new class, you can
  create a new class that inherits the behaviors
  and attributes of the original employee class
• This is reuse at its best

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When you use inheritance you

• Save time because the Employee fields and methods
  already exist
• Save money because it takes less time to create a
  class that uses structure in old classes
• Reduce errors because the Employee methods
  already have been used and tested
• Reduce the amount of new learning required to use
  the new class, because you have used the Employee
  methods on simpler objects and already understand
  how they work

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Base classes

• Classes from which other classes inherit
  attributes or methods
• Classes that inherit from base classes are called
  derived classes
• A base class is also called a superclass
• A derived class is also called a subclass
• You can also use the terms parent class and child
  class

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• public class ASubClass extends ASuperClass
• public ASubClass()
• System.out.println("In subclass
  constructor")

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• public class ASuperClass
• public ASuperClass()
• System.out.println("In superclass
  constructor")

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Extending Classes

• You use the keyword extends to achieve
  inheritance in Java
• Example
• Public class EmployeeWithTerritory extends
  Employee
• You used the extends JApplet throughout Chapters
  9 and 10every JApplet that you wrote is a child
  of the JApplet class

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Object instantiation

• You instantiate an object with a statement such
  as
• employeeWithTerritory northernRep new
  EmployeeWithTerritory()
• Inheritance is a one-way propositiona child
  inherits from a parent, not the other way around
• For example an employee object cannot inherit
  from the employeeWithTerritory subclasscant
  access its methods

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Getting field values from an object

• You can use any of the next statements to get
  field values for the northernRep object
• northernRep.getEmpNum()
• northernRep.getEmpSal()
• northernRep.getEmpTerritory()

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After the northernRep object is declared

• Any of the following statements are appropriate
• northernRep.setEmpNum(915)
• northernRep.setEmpSal(210.00)
• northernRep.setEmpTerritory(5)
• The northernRep object has access to all the
  parent Employee class set methods, as well as its
  own classs new set method

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Child classes are more specific

• An Orthodontist class and Periodontist class are
  children of the Dentist parent class.
• The Dentist class does not have a Orthodontists
  applyBraces() method or the Periodontists
  deepClean() method.
• However, Orthodontist objects and Perodontist
  objects have access to the more general Dentist
  methods

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Instanceof

• Instances of child classes are also instances of
  the parent classes of that class
• The following are true
• If(myOrthodontist instanceof Dentist)
• If(myOrthodontist instanceof Orthodontist)

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Overriding Superclass Methods

• When you extend a superclass, you create a
  subclass that inherits the superclass attributes
  and methods
• What do you do if you do not want these
  attributes and methods?
• You can write your own
• In the musical instruments world a play() method
  would be very different for a guitar as compared
  to a drum
• This is called polymorphism

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Polymorphism

• Literally means many forms
• Consider an employee superclass with a
  printRateOfPay() method
• For weekly employees the print statement might be
• System.out.println(Pay is rateOfPay per
  week)
• For hourly employees the print statement might be
• System.out.println(Pay is rateOfPay per
  hour)

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What do you do?

• When you create a method in a child class that
  has the same name and argument list as a method
  in its parent class, you override the method in
  the parent class
• When you use the method name with a child object,
  the childs version of the method is used

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As an alternative

• You could create and use a method with a
  different name
• But the classes are easier to write and
  understand if you use one reasonable name for
  methods that do essentially the same thing.
• In the example above, because we are attempting
  to print the rate of pay for each object,
  printRateOfPay() is an excellent method name for
  this function.

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Can you think of any other reasons why
polymorphism makes sense?

• When you have to change some aspect of behavior
  that is common to all of the classes and that
  piece of behavior is inherited, then you have to
  change it only in one placenot ten places

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Understanding how Constructors are called During
Inheritance

• Consider the following
• SomeClass anObject new SomeClass()
• Here you are instantiating an object of a
  subclass by invoking the SomeClass() constructor
• You are actually calling at least two
  constructors the constructor for the base class
  and the constructor for the extended, derived
  class.
• When a subclass constructor executes, the
  superclass constructor must execute first and
  then the subclass constructor

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More

• Often the execution of the superclass constructor
  is transparentnothing call attention to the fact
  that the superclass constructor is executing.

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Question

• Suppose that HourlyEmployee is a subclass of
  Employee.
• When you create an object of HourlyEmployee
  called clerk
• What happens in terms of the constructors
  involved???

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• public class ASuperClass
• public ASuperClass()
• System.out.println(In superclass constructor)
• public class ASubClass extends ASuperClass
• public ASubClass()
• System.out.println(In subclass constructor)
• public class DemoConstructors
• public static void main(String args)

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The above produces the following output at the
Command Prompt

• C\Java\java DemoConstructors
• In superclass constructor
• In subclass constructor

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Using Superclass Constructors that Require
Arguments

• When you create a class and do not provide a
  constructor, Java automatically supplies you with
  a default constructorone that never requires
  arguments
• When you write your own constructor, you replace
  the automatically supplied version

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More on constructors

• When all superclass constructors have
  constructors that require arguments, you must
  make sure the subclass constructors provide those
  arguments
• In this case there is no default superclass
  constructor without args

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More on constructors

• Your subclass constructors can contain any number
  of statements, but the first statement within
  each subclass constructor must call the
  superclass constructor
• The format for the statement that calls a
  superclass constructor is
• super(list of arguments)

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Accessing Superclass Methods

• When two methods use the same name in both a
  superclass and a subclass, the sublcass method
  overrides the supperclass method.
• When you want the superclass method to be used,
  you use the keyword super to access the
  superclass method

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• public class Customer
• private int idNumber
• private double balanceOwed
• public Customer(int id, double bal)
• idNumber id
• balanceOwed bal
• public void display()
• System.out.println("Customer "
  idNumber
• " Balance " balanceOwed)

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• public class PreferredCustomer extends Customer
• double discountRate
• public PreferredCustomer(int id, double bal,
  double rate)
• super(id, bal)
• discountRate rate
• public void display()
• super.display()
• System.out.println("Discount rate is "
  discountRate)

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• public class TestCustomers
• public static void main(String args)
• Customer oneCust new Customer(124,
  123.45)
• PreferredCustomer onePCust new
• PreferredCustomer(125, 3456.78, 0.15)
• oneCust.display()
• onePCust.display()

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Command Prompt

• C\JavagtJava TestCustomers
• Customer 124 Balance 123.46
• Chstomer 125 Balance 3456.78
• Discount rate is 0.15
• C\Javagt

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• public class DemoConstructors
• public static void main(String args)
• ASubClass child new ASubClass()

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Learning about Information Hiding

• Information hiding is a way to disallow certain
  attributes and methods to be accessible within
  other classes
• We use the keyword private to make an attribute
  or method local to the class and not accessible
  elsewhere

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• public class Student
• private int idNum
• private double gpa
• public int getIdNum
• return idNum
• public double getGpa()
• return gpa
• public void setIdNum(int num)
• idNum num
• public void setGpa(double gradePoint)
• gpa gradePoint

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Why wont the following code work??

• Suppose you write a main() method in another
  class that does the following
• Student someStudent new Student()
• someStudent.idNum 812
• Only methods contained within the student class
  are allowed to alter Student data
• someStudent.setIdNum(812)

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Remember

• The methods in a subclass can use all the fields
  (attributes, data) in an inherited superclass as
  well as its methods, except.
• Those declared as.

private
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This is called .
Information Hiding
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Are there other access modifiers??

• Yes, Virginia, there are
• Suppose that you want data to be accessible to
  the class it was defined in as well as subclasses
  that extend that class, but not in any other
  classesthat is you dont want the data to be
  public
• Then, you use the keyword protected

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Using Methods you cannot Override

• The three types of methods that you cannot
  override in a subclass are
• static methods
• final methods
• Methods within final classes

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A Subclass Cannot Override static Methods in its
Superclass

• A subclass cannot override methods that are
  declared static in the superclass.
• Not even with the keyword super
• See code below in which ProfessionalBaseballPlayer
  extends BaseballPlayer

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• public class BaseballPlayer
• private int jerseyNumber
• private double battingAvg
• public static void printOrigins()
• System.out.println("Abner Doubleday is often
  "
• "credited with inventing baseball")

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• public class ProfessionalBaseballPlayer extends
  BaseballPlayer
• double salary
• public void printOrigins()
• BaseballPlayer.printOrigins()
• System.out.println("The first professional "
  
• "major league baseball game was played
  in 1871")

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In the above two frames of code..

• Java does not allow the printOrigins() in the
  subclass ProfessionalBaseballPlayer to override
  the static method printOrigins() in the
  superclass BaseballPlayer
• This doesnt work even if you declare the method
  printOrigins() in the subclass ProfessionalBasebal
  lPlayer to be static, as follows

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• public class ProfessionalBaseballPlayer extends
  BaseballPlayer
• double salary
• public static void printOrigins()
• super.printOrigins()
• System.out.println("The first professional "
  
• "major league baseball game was played
  in 1871")

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• However, the following code will work

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• public class ProfessionalBaseballPlayer extends
  BaseballPlayer
• double salary
• public static void printOrigins()
• BaseballPlayer.printOrigins()
• System.out.println("The first professional "
  
• "major league baseball game was played
  in 1871")

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When used with the following class

• Public class TestProPlayer
• public static void main(String args)
• professionalBaseballPlayer aYankee new
  ProfessionalBaseballPlayer()
• aYankee.printOrigins()

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The following output will be produced

• C\JavagtJava TestProPlayer
• Abner Doubleday is often credited with inventing
  baseball
• The first professional major league baseball game
  was played in 1871

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A Subclass Cannot Override final Methods in its
Superclass

• A subclass cannot override methods that are
  declared final in the superclass.
• Consider the classes BasketballPlayer and
  ProfessionalBasketballPlayer below
• These will generate an error at compile time

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• public class BasketballPlayer
• private int jerseyNumber
• public final void printMessage()
• System.out.println("Michael Jordan is the "
  
• "greatest basketball player - and that is
  final")

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• public class ProfessionalBasketballPlayer extends
  BasketballPlayer
• double salary
• public void printMessage()
• System.out.println("I have nothing to say")

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• Public void display(BasketballPlayer bbplayer)
• bbplayer.printMessage()
• The above will cause the code created for
  printMessage() to be inserted inline and the
  practice is called inlining.

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A Subclass Cannot Override Methods in a final
Superclass

• You can also declare a class to be final
• Then all of its methods will be final as well,
  regardless of what access modifiers precede the
  method name in the melthod header

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• public final class HideAndGoSeekPlayer
• private int count
• public void printRules()
• System.out.println("You have to count to "
  count
• " before you start looking for hiders")

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• public final class ProfessionalHideAndGoSeekPlayer
  
• extends HideAndGoSeekPlayer
• private double salary

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• The above two frames of code will generate a
  compiler error when you try to compile the
  ProfessionalHideAndGoSeekPlayer class.

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The end
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Creating a Subclass Method that Overrides a
Superclass Method
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Understanding the role of Constructors in
Inheritance
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Understanding Inheritance when the Superclass
Requires Constructor Arguments
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Accessing an Overridden Superclass Method from
within a Subclass
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Understanding the protected Access Modifier
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