1. Introduction to Classes and Objects Initializing Objects Making Use of Classes in Algorithms Class Examples

2. Introduction to Classes and Objects

3. The Scenario We want to provide a “black box” approach to reusable components… Don’t know how it works, but it works! InputOutput

4. The Scenario We want to create reusable components for other programmers to use. They are focused on their work and don’t want to know the details of how our component works. They are also busy trying to do their work, so maybe they’ll take shortcuts and “hack.” We want to provide services to them but protect the implementation details of our components.

5. Classes A class is the fundamental unit of object-oriented programming. It allows us to implement ADTs, combining both data and operations into one logical bundle.

6. Objects Once we have a class defined, we can then declare any number of objects of that class. For example: Once we have a queue class defined, we can then declare any number of queue objects. Each queue object will then have all the data and procedural abstractions needed for a queue.

7. Classes and Objects A class is like a blueprint: An object is an instantiated class (like a building):

8. Classes vs. Objects Class refers to the template by which we implement an ADT’s functionality. Thus, it is analogous to data type: it is only a definition or template. Object refers to a specific instance of the class. Thus, it is analogous to variable: it is an instance of a template. Class : Object :: Type : Variable

9. Classes Allow for Reuse! Algorithms which make use of the queue class (by declaring instances of the queue, or objects) are clients who obtain services: algorithm Movie_Theatre algorithm Restaurant algorithm Check-out_Line The authors of these algorithms can instantiate and use queues without writing them!

10. Components of a Class The individual data elements of a class are referred to as attributes of the class. The entire collection of data maintained within an object is referred to as the state of the object. The operations (procedures and functions) that are provided by the class (and thus allowed on the data) are called the methods of the class.

11. Controlling Visibility Recall we want to protect the implementation of the class. –Users shouldn’t need to know how it works –Users shouldn’t be able to violate the behaviors (may only do what we allow them). Encapsulation allows us to hide the implementation details.

12. Public vs. Protected Public Protected

13. Visibility Within a Class Public Section (Interface) Protected Section (Implementation) The rest of the algorithm

14. Anatomy of a Class Classes have two sections: Public - the specification of the "visible” part of the class Protected - the specification of the "hidden” part of the class

15. Public Section Contains everything that the client algorithm needs to know to use the class Is the visible part of the class Defines the interface and contract with the user (algorithm) Contains the header lines of those methods that are available for clients of the class to call.

16. Protected Section Contains the details of the implementation of the class Cannot be seen by the client algorithm Hides attributes and method implementation Contains all method declarations and implementations.

17. Interactions Protected Data & Implementation of Modules Public Module Definitions and Contract Client Algorithm Supplier Object

18. Encapsulation and Hiding A client algorithm can see only the public section of the class. A client has no idea what is within the protected section. A client algorithm can interact with an object only by calling the methods listed in the public section of the class. A client algorithm cannot directly access any of the data within an object. It may “get at” data only via calls to public methods.

19. class data public method 1 public method 2 public method 3 a protected method Public vs. Protected Methods Clients may call any/all of the three public methods. Clients don’t know the hidden method exists; they cannot call it. Some Class

20. Protected Methods A protected method is one which is defined in the protected section that does not have its header line in the public section. Clients don’t even know it exists. Such a method may only be called by other methods within the protected section.

21. Structure of a Class Definition class public protected endclass //

22. A Bus Class Example MAX_PASSENGERS Current Initialize Add Passengers Remove Passengers Number of Passengers Bus Implementation

23. class Bus public procedure Initialize // contract here procedure AddPassengers (to_add iot in num) // contract here function NumPassengers returnsa num // contract here procedure RemovePassengers (to_remove iot in num) // contract here protected MAX_PASSENGERS is 50 current isoftype num procedure Initialize current <- 0 endprocedure // Initialize

24. procedure AddPassengers (to_add iot in num) if (current + to_add <= MAX_PASSENGERS) then current <- current + to_add endif endprocedure // AddPassengers function NumPassengers returnsa num NunPassengers returns current endfunction // NumPassengers procedure RemovePassengers (to_remove iot in num) if (current >= to_remove) then current <- current – to_remove endif endprocedure // RemovePassengers endclass // Bus

25. Recipe for Writing a Class Write shell/template Decide on what the user needs to know about - PUBLIC Specify the protected data Diagram & think of sample algorithm Write public section with contract information Write protected section with implementation details

26. Attribute Visibility in the Protected Section Variables declared at the global level of the protected section can be directly accessed throughout the protected section without passing them via parameter. protected current isoftype num... procedure AddPassengers (to_add iot in num) if (current + to_add <= MAX_PASSENGERS) then current <- current + to_add endif endprocedure // AddPassengers function NumPassengers returnsa num NunPassengers returns current endfunction // NumPassengers...

27. Attribute Visibility in the Protected Section Allow direct access, bypassing parameters! May want clients of the class to be able to call methods Don't want clients to know the details of the protected section's data representation. If method parameter lists included data that's protected, then would be telling the client about that representation. So, allow direct access within protected Still have encapsulation (the class itself)

28. Summary Classes implement ADTs An object is an instance of a class Encapsulation allows for the protection of details within the class –Public section provides interface –Protected section provides implementation Attributes declared at the global level of the protected section may be directly accessed within any method in the protected section.

29. Questions?

30. Initializing Objects

31. The Scenario You’ve defined a Queue class and now want to create an instance of that class. There are some attributes which should be initialized when an object is created. We’d like to initialize our Head and Tail pointers to NIL. But the algorithm shouldn’t know about the pointers!

32. Need to Initialize Our Queue class will thus need a method that “sets things up” when an object is created. Different languages handle this differently. We’ll use a normal method that must be invoked like any other…

33. “A Method By Any Other Name…” An initialization method is just like any other method. For clarity and convenience, we always use the identifier Initialize for such methods.

34. Initialize Common but Not Required Most classes have attributes that require some initial values. We expect most classes to have initialization methods. But it’s not required.

35. Initialize In the Class Definition class Initialize_Example public... procedure Initialize ( ) // contract information... protected... procedure Initialize ( ) endprocedure // Initialize endclass

36. Invoking the Initialize Method In the algorithm, we invoke (or call) the Initialize method just as any other method. It is assumed that Initialize is only called once, but there is no rule to enforce this. The algorithm must explicitly call the initialize method for each object that has attributes that need to be initialize.

37. An Example In the algorithm: MyQueue isoftype Queue MyQueue.Initialize In the class definition: procedure Initialize head <- NIL tail <- NIL endprocedure // Initialize

38. Another Example In the algorithm: MyAirplane isoftype Plane MyAirplane.Initialize(“Delta 955”) In the class definition: procedure Initialize(flight_name iot in string) altitude <- 0 current_flight <- flight_name endprocedure // Initialize

39. Summary Very often, objects have attributes that need some initial values. Thus these classes need some method to set these attributes. Different languages handle this differently: –Some have “special” methods that are automatically executed. –We’ll treat the Initialize method like any other method and call it from the algorithm for each object created.

40. Questions

41. Making Use of Classes in Algorithms

42. The Scenario You need to create a simulation of an airport. A coworker has developed an airplane class you’d like to use in your algorithm. How do we “get at” the airplane class? Notice we don’t have access to modify the class, just make use of it.

43. Importing the Class Algorithms and class definitions often reside in separate computer files. –Recall that a benefit of Object-Oriented Programming is re-use via class libraries We need the ability to import a class into our algorithm or another class The “uses” clause allows this import.

44. The “Uses” Clause algorithm uses,,...... endalgorithm // class uses... endclass //

45. An Example Having previously defined an Airplane class, the algorithm can make use of the class and create objects of the class as follows: algorithm Airport uses Airplane Cessna, Prop isoftype Airplane...

46. A More Complex Example algorithm BigAirport uses Airplane, Helicopter, FlightController Plane1, Plane2 isoftype Airplane ControlDeck isoftype FlightController... endalgorithm

47. Classes and Objects A class is like a blueprint: It is analogous to data type An object is an instantiated class (like a building): It is analogous to variable Class : Object :: Type : Variable

48. Creating Objects Once the algorithm has imported the class definition, then it can create objects of the class. Objects are created just as any variable: isoftype For example: MyPlane, YourPlane isoftype Airplane ControlDeck isoftype FlightController

49. Accessing Methods of Objects After instantiating a class and creating an object in the algorithm, we want to make use of its methods. There are many methods in the class, so how do we access the one we want? Just as fields of a record, we access the methods in an object using the dot (.) operator.

50. Invoking Methods of an Object algorithm Example uses MyClass MyObject isoftype MyClass MyObject.Initialize MyObject.... endalgorithm

51. An Airplane Class Airplane Initialize TakeOff ChangeAltitude InTheAir Altitude IsFlying ServeSnack Land Fly

52. An Example of Accessing Methods algorithm Flying uses Airplane MyAirplane isoftype Airplane MyAirplane.Initialize if (NOT MyAirplane.IsFlying) then MyAirplane.TakeOff MyAirplane.ChangeAltitude(30000) MyAirplane.Fly(“Cincinnati”) MyAirplane.Land else print(“Sorry, that plane is already flying!”) endalgorithm Function Procedures

53. Only Access Public Methods The algorithm cannot access protected attributes or protected methods. The algorithm only has access to methods as declared in the public section. Examples of illegal use: MyAirplane.Altitude <- 30000 print(MyAirplane.InTheAir) MyAirplane.ServeSnack // method call

54. Summary Import class definitions using the uses clause. Declare objects as any other variable. Access public methods of objects using the dot (.) operator.

55. Questions?

56. Class Examples Airplane, Queue, Pile

57. Once Written, It’s Easy! Once we’ve written the class… We test it and validate that it works We can then make use of it in any algorithm Notice in the following algorithm examples how little work is done –All manipulation is hidden from the algorithm –All the “details” are abstracted into the object

58. Airplane Example An Airplane knows how to: Take off Land Fly to a destination (and serve a snack) Change its altitude It also has the following attributes Current altitude Whether it’s flying or not

59. Airplane Symbolic Diagram Airplane Initialize TakeOff ChangeAltitude InTheAir Altitude IsFlying ServeSnack Land Fly

60. class Airplane public procedure TakeOff // comments here procedure Land // comments here procedure ChangeAltitude (NewHeight iot in Num) // comments here function IsFying returnsa boolean // comments here procedure Initialize // comments here procedure Fly (destination iot in String) // comments here protected // create the persistent data InTheAir isoftype Boolean Altitude isoftype Num

61. // still in the protected section procedure Initialize InTheAir <- FALSE Altitude <- 0 endprocedure // Initialize procedure TakeOff if InTheAir then print("I'm in the air!") else InTheAir <- TRUE ChangeAltitude(3000) endif endprocedure // TakeOff

62. // still in the protected section procedure ChangeAltitude (NewHeight iot in Num) Altitude <- NewHeight endprocedure // ChangeAltitude procedure Fly (destination iot in String) print(“I’m flying to”, destination) ServeSnack endprocedure // Fly procedure ServeSnack // comments here MAKE PASSENGERS HAPPY endprocedure // ServeSnack

63. // still in the protected section function IsFlying returnsa boolean IsFlying returns InTheAir endfunction // IsFlying procedure Land if InTheAir then InTheAir <- FALSE ChangeAltitude(0) else print("I'm not in the air!") endif endprocedure // Land endclass // Airplane

64. Using the Airplane Class algorithm Airport uses Airplane Cessna1090 isoftype Airplane Cessna1090.Initialize Cessna1090.Takeoff Cessna1090.ChangeAltitude(30000) Cessna1090.Fly(“Baltimore”) Cessna1090.Land endalgorithm

65. The Queue A collection with restricted set of operations to change its state: only modified by adding to one end and deleting from the other. Enqueue Dequeue

66. NumberQueue Symbolic Diagram NumberQueue Initialize Enqueue Dequeue head tail IsEmpty … IsFull

67. class NumberQueue public procedure Enqueue(value iot in Num) // contract information here procedure Dequeue(value iot out Num) // contract - queue not empty procedure Initialize // contract information here function IsEmpty returnsa Boolean // contract information here function IsFull returnsa Boolean // contract information here protected List_type definesa record data isoftype Num next isoftype Ptr toa List_type endrecord // create the persistent data head, tail isoftype Ptr toa List_type

68. // still in the protected section procedure Enqueue(value iot in Num) temp isoftype Ptr toa List_type temp <- new(List_type) temp^.data <- value temp^.next <- NIL if(IsEmpty) then head <- temp else tail^.next <- temp endif tail <- temp endprocedure // Enqueue

69. // still in the protected section procedure Dequeue (value iot out Num) if(IsEmpty) then // violates contract! Error! else value <- head^.data head <- head^.next if(IsEmpty) then tail <- NIL endif endprocedure // Dequeue

70. // still in the protected section function IsEmpty returnsa Boolean IsEmpty returns (head = NIL) endfunction // IsEmpty function IsFull returnsa Boolean IsFull returns FALSE // dynamic endfunction // IsFull procedure Initialize // initialize the persistent data head <- NIL tail <- NIL endprocedure // Initialize endclass // NumberQueue

71. algorithm Store uses NumberQueue temp isoftype num checkout isoftype NumberQueue checkout.Initialize... loop some people enter and leave store randomly exitif ((no people in store) AND (closing_time)) if (someone walks up for service) then checkout.Enqueue(person’s number) endif if (NOT checkout.IsEmpty) then checkout.Dequeue(temp) print(“Now servicing person”, temp) endif endloop endalgorithm // Store

72. Example: Simulating the Lotto We want to define a class that will allow us to simulate the lottery. We want to place elements into random locations in the collection. When we get an item from the collection, we want a random element.

73. A “Pile” Class A data structure in which –Items are inserted somewhere randomly in the middle of the structure –Items are removed from a random location in the structure

74. Pile Symbolic Diagram NumPile Initialize StickOn DigOut Head num_of_things IsEmpty Random …

75. class NumPile public procedure StickOn (the_thing iot in Num) //purpose: put an item on the pile. //pre: none //post: the pile has the item added to it procedure DigOut (the_thing iot out Num) //purpose: get an item off of the pile. //pre: the pile is not empty. // post: the pile has a random element // removed. function IsEmpty returnsa boolean // comments here - contract procedure Initialize // comments here - contract

76. protected PileNode definesa Record thing isoftype Num next isoftype ptr to PileNode endrecord // PileNode head isoftype ptr toa PileNode num_of_things isoftype Num procedure Initialize num_of_things <- 0 head <- NIL endprocedure // Initialize function IsEmpty returnsa boolean IsEmpty returns (head = NIL) endfunction // IsEmpty

77. // still in the protected section function Random returnsa Num // returns a random number <= // num_of_things endfunction // Random procedure StickOn (thing isoftype in Num) place_to_insert isoftype Num place_to_insert <- Random new_node isoftype ptr toa PileNode new_node <- new(PileNode) // loop through pile until place-to- // insert is reached, then insert node num_of_things <- num_of_things + 1 endprocedure // StickOn

78. // still in the protected section procedure DigOut (thing isoftype out Num) thing_to_snag isoftype Num place_to_get isoftype Num place_to_get <- Random // code for looping through pile to // find right thing-to-snag, then // remove it num_of_things <- num_of_things - 1 thing <- thing_to_snag endprocedure // Dig-Out endclass // NumPile

79. Using the Pile Class algorithm Lotto uses NumPile lotto_pile isoftype NumPile lotto_pile.Initialize ticket isoftype Num loop exitif (All Entries Purchased) Get_Entry(ticket) // get input from user lotto_pile.StickOn(ticket) endloop // Now, find one winner lotto_pile.DigOut(ticket) print ("The winning number is", ticket) endalgorithm // Lotto

80. Summary Writing classes involves considerable work in –Design, Implementation, & Testing But once done, algorithms may make use of the classes –Instantiating objects and manipulating them –Hiding the details and implementation –Much of the work is done inside the object

81. Questions?