Inheritance and Design

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Inheritance and Design

• CSIS 3701 Advanced Object Oriented Programming

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Key Questions

• Given list of required classes
• What classes should extend others?
• Are additional classes needed?
• What methods can be inherited/overridden?
• Do methods need to be designed for polymorphism?

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Employees Example

• Goal Payroll system prints weekly paychecks
• Paycheck of form Pay for name amount
• Also tracks total pay for all employees
• Should store information in single data structure
• Problem Many types of workers paid in different
  ways

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Employees Example

• Employee types
• Salaried Yearly salary / 52
• Manager Like salaried, with additional weekly
  bonus
• Intern Paid hourly rate for hours worked (may be
  different every week). Cannot work
  over 40 hours.
• Consultant Paid hourly rate for hours worked
  (may be different every
  week), with additional travel
  allowance

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Polymorphism and Class Design

• Is polymorphism necessary?
• All objects in single container or manipulated by
  single method (add method in JPanel)
• What methods must be polymorphic?
• Will be iterated for all objects in container,
  etc.
• Must be defined in superclass
• Example
• All employees stored in single array
  (polymorphism needed)
• Will call getPay() and getCheck() for each

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Hierarchy Design

• Inheritance hierarchy often tree structure
• Required classes often leafs
• Overall superclass at root
• Abilities inherited along branches

?????
Consultant
Intern
Salaried
Manager
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Superclass Identification

• What abilities/information are common to all
  required classes?
• Defines overall superclass at root of hierarchy
• Example All employees have
• A name
• Ability to compute weekly pay
• Ability to print name/pay on paycheck

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Superclass Identification

• Employee superclass

Employee String name double getPay() String
getCheck()
Consultant
Intern
Salaried
Manager
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Abstract Classes

• Problem Users should not be able to create
  instances of Employee
• Only exists to define common features of actual
  required classes
• Will not actually have all required abilities for
  use
• Employee e new Employee()
• double p e.getPay()

This is not defined for generic employee
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Abstract Classes

• Can define class as abstract
• Cannot be instantiated
• Very common in JavaExample abstract JComponent
  class
• Superclass of all visual swing components
• Defines common properties of x, y, width, height
• public abstract class Employee
• private String name
• public Employee(String n) name n

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Abstract Methods

• Often need to declare a method in superclass
  without actually defining it
• Example getPay() method in Employee class
• No way to provide actual definition at this level
• Necessary if polymorphism required
• Employee e new Salaried(Fred, 26000)
• pay e.getPay()

Will not work unless getPay() in Employee class
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Abstract Methods

• Can declare method abstract in superclass
• Must be overridden in subclasses
• Often used to enable polymorphism
• Form like C prototypes
• public abstract returntype methodname(params)

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Final Employee Superclass

• public abstract class Employee
• private String name
• public Employee(String n)name n
• public abstract double getPay()
• public String getCheck()
• return "Pay for "name" \t" getPay()
• Can still define getCheck even if calls abstract
  method
• Will only be invoked on subclass objects
• Those will use their overridden version of
  getPay()

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Hierarchical Decomposition

• What do some classes have in common?
• Salaried and Manager have salary
• Intern and Consultant have hours worked, rate of
  pay, and method to set hours
• What must those classes do differently?
• Manager also has bonus
• Consultant also has travel allowance
• Intern must validate that hours 40

Must look at data and methods
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Hierarchical Decomposition

• Are the abilities of one class a subset of
  another?
• One class has all the same abilities as another,
  and some additional abilities
• If so, make C2 subclass of C1

additional
data method
data method

C2
C1
data methoddata method data method
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Supervisor/Manager Example

• Salaried
• Stores name (inherited from Employee), salary
• Computes pay as salary/52
• Manager
• Stores name , salary, and bonus
• Computes pay as salary/52 bonus
• Manager should be subclass of Salaried

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Hierarchical Decomposition

• Do abilities of different classes intersect
• Classes have shared abilities, but neither subset
  of other
• If so, create abstract class with shared abilites
• Both classes now extend it

data method shared data data method
shared method
data method
data method
C1
C2
New abstract C3
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Hierarchical Decomposition

• Commonly done in Java
• Example JTextComponent class

JComponent int x, y, width, height
Common to all visual components
JTextComponent String text String getText() void
setText(String)
Common to visual components that accept text
JTextField
JTextArea
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Consultant / Intern Example

• Consultant
• Stores name, hours, rate, travel
• Computes pay as hours rate travel
• Has method to set hours
• Intern
• Stores name, hours, rate
• Computes pay as hours rate
• Has method to set hours which must validate 40
• Create Hourly class with shared abilities

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Consultant / Intern Example
Hourly int hours, double rate set by
constructor getPay() hours rate setHours(h)
hours h
Consultant double travel getPay() superclass
version travel
Intern setHours(h) check h lt 40
before calling superclass version
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Final Hierarchy Design
Employee (abstract)
Hourly (abstract)
Salaried
Manager
Intern
Consultant
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Method Design

• Design methodologies
• Subclass methods should only directly manipulate
  subclass variables
• Subclass methods should (if possible) call
  superclass version to manipulate inherited
  superclass variables

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Supervisor/Manager Example
Salaried double salary getPay() salary/52
Manager double bonus getPay() salary/52 bonus
superclass version of getPay
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Salaried Class

• public class Salaried extends Employee
• private double salary // Yearly salary
• public Salaried(String n, double s)
• super(n)
• salary s
• public double getPay()
• return salary/52

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Manager Class

• public class Manager extends Salaried
• private double bonus
• public Manager(String n, double s, double b)
  
• super(n, s)
• bonus b
• public double getPay()
• return super.getPay() bonus

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Hourly Class

• public abstract class Hourly extends Employee
• private double rate // Hourly pay rate
• private int hours // hours worked this
  week
• public Hourly(String n, double r)
• super(n)
• rate r
• hours 0
• public void setHours(int h)
• hours h
• public double getPay()
• return hours rate

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Consultant Class

• public class Consultant extends Hourly
• private double travel // Travel allowance
• public Consultant(String n, double r, double t)
  
• super(n, r)
• travel t
• public double getPay()
• return super.getPay() travel

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Intern Class

• public class Intern extends Hourly
• public Intern(String n, double r)
• super(n, r)
• public void setHours(int h)
• if (h gt 40)
• h 40
• super.setHours(h)

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Encapsulation

• Access to superclass variables may not be
  possible with public methods
• superclass should provide protected methods to
  allow access to superclass variables
• Example
• Intern class must print message with employee
  name if error
• Employee must provide protected method to allow
  access to name

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Encapsulation

• public abstract class Employee
• private String name
• protected String getName() return name
• public class Intern extends Hourly
• public void setHours(int h)
• if (h gt 40)
• System.out.println(getName()
  can only work 40 hours)
• else super.setHours(h)

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Polymorphism

• Create objects in different classes
• public static void main(String args)
• Salaried wally new Salaried("Wally", 52000)
• Salaried alice new Salaried("Alice", 78000)
• Manager phb new Manager("Pointy Haired Boss",
  104000, 200)
• Intern asok new Intern("Asok", 20)
• asok.setHours(50)
• Consultant dogbert new Consultant("Dogbert",
  100, 500)
• dogbert.setHours(42)

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Polymorphism

• Store in single data structure
• Employee workers new Employee5
• workers0 phb
• workers1 alice
• workers2 wally
• workers3 asok
• workers4 dogbert

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Polymorphism

• Manipulate as though instances of superclass
• double total 0
• for (int i 0 i lt 5 i)
• System.out.println(workersi.getCheck())
• total workersi.getPay()
• System.out.println("Total pay \t" total)