CS 112 Introduction to Programming

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CS 112 Introduction to Programming

• Lecture 15
• Defining Classes
• Http//zoo.cs.yale.edu/classes/cs112/

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Outline

• Admin. and review
• Classes and objects
• Defining classes
• Basic syntax
• Examples

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Admin.

• Assignment 3
• Due Oct. 13
• Two problems
• Problem 1 Control structure and method reuse
• Problem 2 Method decomposition

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Review Program Development Process
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Design
year GetYearFromUser()
month GetMonthFromUser()
month ? 0
t
f
PrintMonth(month, year)
PrintYear(year)
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Design PrintMonth( month, year )
Print month header
GetFirstWeekday (month, year)
IndentInitialWeekdays
GetMonthDays(month, year)
Print each day
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Methods To Be Implemented and an Order to
Implement them

• static string GetMonthName( int month )
• // The English word for a specific month
• static bool IsLeapYear( int month, int year )
• // Check if a year is a leap year
• static int GetMonthDays( int month, int year )
• // Calculate the number of days for a specific
  month of a specific year
• static int GetFirstWeekday( int month, int year
  )
• // Calculate the weekday of the first day of a
  month (in a specific year)
• static void IndentInitialWeekdays( int weekday )
• // Indent the first line so that the first
  day appears in the correct position
• static void PrintMonth( int month, int year )
• static void PrintYear( int month, int year )
• static void GievInstructions( int month, int year
  )
• static void GetYearFromUser( int month, int year
  )
• static void GetMonthFromUser( int month, int year
  )
• static void Main( string args )

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Example Implementing GetMonthName

• static string GetMonthName(int month)
• switch (month)
• case 1 return ("January")
• case 2 return ("February")
• case 3 return ("March")
• case 4 return ("April")
• case 5 return ("May")
• case 6 return ("June")
• case 7 return ("July")
• case 8 return ("August")
• case 9 return ("September")
• case 10 return ("October")
• case 11 return ("November")
• case 12 return ("December")
• default return ("Illegal month")
• // end of method GetMonthName

static void Main( string args )
Console.WriteLine( GetMonthName( 1 ) ) //
end of method Main
Test
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Example Implementing IsLeapYear

• static bool IsLeapYear(int year)
• return ( (year 4 0) (year 100 ! 0)
  )
• ( year 400 0)
• // end of method IsLeapYear

static void Main( string args )
Console.WriteLine( IsLeapYear( 2000 ) )
Console.WriteLine( IsLeapYear( 1900 ) )
Console.WriteLine( IsLeapYear( 2003 ) ) // end
of method Main
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Example Implementing GetMonthDays

• static int GetMonthDays(int month, int year)
• switch ( month )
• case 2
• return IsLeapYear( year ) ? 29 28
• case 4 case 6 case 9 case 11
• return (30)
• default
• return (31)
• // end of switch
• // end of method GetMonthDays

static void Main( string args )
Console.WriteLine( GetMonthDays( 2, 2000 ) )
Console.WriteLine( GetMonthDays( 2, 1900 ) )
Console.WriteLine( GetMonthDays( 6, 2003 )
) // end of method Main
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Summary Method Control Flow
PrintMonth
GetMonthDays


GetMonthDays()



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Summary

• Design
• Top-down stepwise refinement
• Implementation and testing
• Bottom-up
• Write the simple functions first
• Write a piece and test a piece
• Test a method
• using some dummy method calls, e.g.
• Call the method in Main() with some typical
  test cases
• using Visual Studio Immediate window

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Outline

• Admin. and review
• Classes and objects
• Defining a class
• Basic syntax
• Examples

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Object-Oriented Design

• The focus of methods is on doing things roughly
  speaking, we can say that methods focus on the
  verbs.
• In object-oriented design, we focus on the nouns
• In object-oriented design, we tend to group
  methods together according to the nouns
• Important for large, complex programs

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C Classes

• A C class plays dual roles
• Program module containing a list of (static)
  method declarations and (static) data fields
• Blueprint for generating objects
• It is the model or pattern from which objects are
  created
• Supports two techniques which are essence of
  object-oriented programming
• data encapsulation (for abstraction)
• inheritance (for code reuse)

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User-Defined Class

• A user-defined class is also called a
  user-defined type
• class written by a programmer
• A class encapsulates (wrap together) data and
  methods
• data members (member variables or instance
  variables)
• methods that manipulate data members

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Objects

• An object has
• state - descriptive characteristics
• behaviors - what it can do (or be done to it)
• For example, consider a coin in a computer game
• The state of the coin is its current face (head
  or tail)
• The behavior of the coin is that it can be
  flipped
• Note the interactions between state and behaviors
• the behavior of an object might change its state
• the behavior of an object might depend on its
  state

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Outline

• Admin. and review
• Classes and objects
• Defining a class
• Basic syntax
• Examples

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Defining Classes

• Use Project lt Add Class to add a new class to
  your project
• A class contains data declarations and method
  declarations

Data declarations
Method declarations
Member (data/method) Access Modifiers public
member is accessible outside the class private
member is accessible only inside the class
definition
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Data Declarations

• You can define two types of variables in a class
  but not in any method (called class variables)
• static class variables
• nonstatic variables are called instance variables
  (fields) because each instance (object) of the
  class has its own copy
• class variables can be accessed in all methods of
  the class
• Comparison Local variables
• Variables declared within a method or within a
  block statement
• Variables declared as local variables can only be
  accessed in the method or the block where they
  are declared

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Method Declarations

• A class can define many types of methods, e.g.,
• Access methods read or display data
• Predicate methods test the truth of conditions
• Constructors
• initialize objects of the class
• they have the same name as the class
• There may be more then one constructor per class
  (overloaded constructors)
• can take arguments
• they do not return any value
• it has no return type, not even void

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Outline

• Admin. and review
• Classes and objects
• Defining a class
• Basic syntax
• Examples

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Example Time1 class

• We define the Time1 class to represent time
  midnight to 115959
• The state of a time object can be represented by
• hour, minute, second integers representing time
• We might define the following methods
• a Time1 constructor, to set up the object
• a SetTime method, to set time
• a ToUniversalString method, to convert the
  internal representation to a string representing
  the time in 24 hour format
• a ToStandardString method, to convert the
  internal representation to a string representing
  the time in 12 hour format

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Time1.cs

• 1 // Fig. 8.1 Time1.cs
• 2 // Class Time1 maintains time in 24-hour
  format.
• 3
• 4 using System
• 5
• 6 // Time1 class definition
• 7 public class Time1
• 8
• 9 private int hour // 0-23
• 10 private int minute // 0-59
• 11 private int second // 0-59
• 12
• 13 // Time1 constructor initializes instance
  variables to
• 14 // zero to set default time to midnight
• 15 public Time1()
• 16
• 17 SetTime( 0, 0, 0 )
• 18
• 19

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Time1.cs

• 33 // convert time to universal-time (24
  hour) format string
• 34 public string ToUniversalString()
• 35
• 36 return String.Format(
• 37 "0D21D22D2", hour,
  minute, second )
• 38
• 39
• 40 // convert time to standard-time (12
  hour) format string
• 41 public string ToStandardString()
• 42
• 43 return String.Format(
  "01D22D2 3",
• 44 ( ( hour 12 hour 0 ) ? 12
  hour 12 ),
• 45 minute, second, ( hour lt 12 ? "AM"
  "PM" ) )
• 46
• 47
• 48 // end class Time1

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TimeTest1.cs

• 1 // Fig. 8.2 TimeTest1.cs
• 2 // Demonstrating class Time1.
• 3
• 4 using System
• 5 using System.Windows.Forms
• 6
• 7 // TimeTest1 uses creates and uses a Time1
  object
• 8 class TimeTest1
• 9
• 10 // main entry point for application
• 11 static void Main( string args )
• 12
• 13 Time1 timeObj1 new Time1() //
  calls Time1 constructor
• 14 string output
• 15
• 16 // assign string representation of
  time to output
• 17 output "Initial universal time is "
  
• 18 timeObj1.ToUniversalString()
  
• 19 "\nInitial standard time is
  "

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TimeTest1.cs Program Output

• 34 output "\n\nAfter attempting
  invalid settings "
• 35 "\nUniversal time "
  time.ToUniversalString()
• 36 "\nStandard time "
  time.ToStandardString()
• 37
• 38 MessageBox.Show( output, "Testing
  Class Time1" )
• 39
• 40 // end method Main
• 41
• 42 // end class TimeTest1

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Class View and Object Browser

• Class View and Object Browser are features of
  Visual Studio that facilitate the design of
  object-oriented applications
• Class View
• Displays variables and methods for all classes in
  a project
• Displays as treeview hierarchical structure
• at nodes allows nodes to be expanded
• - at nodes allows nodes to be collapsed
• Can be seen by selecting View lt Class View

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Class View Example
Fig. 8.20 Class View of class Time1 (Fig. 8.1)
and class TimeTest1 (Fig. 8.2).
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Object Browser

• Object Browser
• Lists all classes in a library
• Helps developers learn about the functionality of
  a specific class
• To view the Object Browser select any .NET FCL
  method, right click, and select Go To Definition

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Object Browser Example
Fig. 8.21 Object Browser when user selects
MessageBox from TimesTest1.cs.
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Example 2 The Coin Class

• We define a Coin class to model a coin in a game
• In our Coin class we could define the following
  data
• face, an integer that represents the current face
• HEADS and TAILS, integer constants that represent
  the two possible states
• We might also define the following methods
• a Coin constructor, to set up the object
• a Flip method, to flip the coin
• a GetFace method, to return the current face
• a StateToString method, to return a string
  description of the current state
• See Coin.cs, CountFlips.cs, FlipRace.cs
• To compile CountFlips csc CountFlips.cs
  Coin.cs
• To compile FlipRace csc FlipRace.cs Coin.cs

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Instance Data The Two Coins in FlipRace
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Backup Slides
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Summary Data Scope

• Variables declared in a larger scope can be
  accessed in an enclosed scope, e.g.
• Variables declared in a class can be accessed by
  the methods in the class
• Variables declared in a method can only be
  accessed in the method
• Variables declared in a block can only be
  accessed in the block