Announcements

1
Announcements

• Your resources include
• me (your instructor)
• the TAs and consultants
• cornell.class.cs100
• http//www.cs.cornell.edu/cs100-su99/
• email!
• No class on Monday

2
Todays Topics

• Review
• One whole program
• Inheritance
• Constructors of superclasses
• Access modifier protected
• Overriding methods
• The method toString() for all classes

3
Review

• Public and private?
• What does (s1 s2) indicate if s1 and s2 are
  strings?
• Difference between procedure and function?
• Whats a constructor good for?

4
A Whole Program
Indicate that we want to be be able to use any
class in package java.io Note java.lang is
always automatically imported. Thats where
system.out.println comes from, for example

• import java.io.
• public class Rectangle
• public int width
• public int length
• public Rectangle(int side1, int side2)
• width side1
• length side2
• public int area()
• return(length width)

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Rest of Program

• public class TrivialApplication
• public static void main(String args) throws
  IOException
• BufferedReader stdin new BufferedReader
• (new InputStreamReader(System.in))
• int userWidth 0 int userLength 0
• System.out.println("Width of your rectangle?")
• userWidth Integer.parseInt(stdin.readLine())
• System.out.println("Length of your
  rectangle?")
• userLength Integer.parseInt(stdin.readLine())
  
• Rectangle r new Rectangle(userWidth,
  userLength)
• System.out.print("In a rectangle of width "
  userWidth)
• System.out.print(" and length " userLength
  " the area is ")
• System.out.println(r.area())
• try System.in.read() // prevent console
  window from going away
• catch (java.io.IOException e)

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Deconstruction

• public class TrivialApplication
• public static void main(String args) throws
  IOException
• BufferedReader stdin new BufferedReader
• (new InputStreamReader(System.in))

A class definition
Ignore this for now, used when taking args from
the commandline
Well discuss exceptions later
An object created to help with I/O
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Inheritance -- Briefly

• A subclass B of class A inherits fields and
  methods from class A
• A is a super-class of B
• In Java, the keyword extends is used to define a
  subclass

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Method toString()

• // Yield a string containing the data for //
  that person
• public String toString() return name
  salary hiredate
• Actually, every class implements method toString
• Note in ltstringgt B, if B is not a string,
  B.toString is used to convert it.

9
Recall the class Employee. . .

• // An instance of Employee contains a person's
  name,
• // salary, and year hired. It has a constructor
  and
• // methods for raising the salary, printing the
  data, and
• // retrieving the person's name and the year
  hired.
• public class Employee
• public String name // The person's name
• public double pay // The person's yearly salary
• public int hireDate // The year hired

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• // Constructor a person with name n, salary
• // s and year d hiredpublic Employee(String n,
  double s, int d) name n pay s
  hireDate d
• // Raise the pay by p percent
• public void raiseSalary(double p)
• pay pay (1 p/100.0)
• // Yield the year the person was hired
• public int hireYear()
• return hireDate

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• // Yield the person's name
• public String getName()
• return name
• // Yield a String containing persons data
• public String toString()
• String s name
• s s " " pay " " hireDate
• return s
• This was the old class Employee

12
Todays Task

• Modify class Employee to take into account 3
  different kinds
• VIP -- Need a field bonus. Get a yearly salary
• Salaried -- Expected to work overtime without
  extra pay
• Regular -- Hourly wage instead of yearly salary,
  have time cards. Could record hours worked each
  week, but will assume 40

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How to add these things? Add fields?

• int employeeKind // VIP 1, salaried 2
  // regular 3
• double bonus // bonus for VIP only
• double hourlyWage // for regular emp only
• // method to set bonus if employee is VIPpublic
  setBonus(double b) if (employeeKind 1)
  bonus b
• Many other changes required. . .

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Problems

• Now each employee has a field for bonus and
  hourly wage but not all employees need these
  fields
• Have to test to determine what kind of employee
  is being processed within many of the methods.
  This produces ugly code.
• What if a new kind of employee is added (VVIP)?
  Everything has to change again

15
Better Solution -- Inheritance

• public class VIP extends Employee
• private double bonus // the VIPs bonus
• // Constructor person with name n, // year d
  hired, pay s and bonus b
• public VIP(String n, int d, double s, double b)
  super(n, d) pay s bonus b

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Other Methods in class VIP

• // Yield a string containing datapublic String
  toString() return VIP name pay
  bonus hireDate // Change
  bonus to ppublic void changeBonus(double p)
• bonus p

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Whats Going on?

• An instance of class VIP has
• every field that the class Employee has
• plus the ones VIP declares
• every method that the class Employee has. . .
• . . .except those that are overridden

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VIP extends Employee
Employee x x new Employee(Millett, 1999)
Employee y new VIP(Buck, 1999, 100000,
1000)
VIP
y
Name Buck pay 100000 hireDate 1999
Employee, hireYear, getName, toString bonus
1000 VIP changeBonus
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super

• For an instance of VIP, a constructor of the
  super-class Employee should be called to
  initialize the fields declared in Employee.
• This is the first statement in a constructor for
  VIP super(n, d)
• This of this as equivalent to Employee(n, d)

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Protected/Public/Private

• If fields of Employee are public, they can be
  referenced from anywhere.
• If they are private, they can be referenced only
  from instances of Employee.
• If they are protected, they can be referenced
  only in same package --concept discussed much
  later!!!

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Methods in sub/super-classes

• VIP y
• y new VIP(Perkins, 1983, 90000, 1000)
• y.getName() refers to method getName of its
  superclass, Employee. This is because getName is
  not defined in VIP.
• y.getBonus() refers to method getBonus of VIP.
  This is because getBonus is defined in VIP.
• y.toString() refers to method toString of VIP.
  This is because toString is defined in VIP.
• Method toString of superclass Employee has been
  overridden in class VIP.

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Intuitively, Inheritance. . .

• Derive a new class from an existing class
• Purpose Software re-use
• The is-a relationship. The subclass is a more
  specific version of the superclass
• e.g. VIP is a Employee

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Encapsulation

• Objects are black boxes -- ultimately, we dont
  want to worry about how they work
• Other things interact with object through
  service methods
• An object should be self-governing (variables can
  only be modified w/in the object itself)

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Discussion Questions

• Can source code be censored?
• Who is responsible if a system fails?
• Is programming necessarily a solitary pursuit?