Object Oriented Programming Chapter 7 Programming Languages by Ravi Sethi

1. Object Oriented ProgrammingChapter 7Programming Languagesby Ravi Sethi

2. 7.1 What Is An Object • Object-oriented program - Description or simulation of application • Object-oriented programming is done by adopting or extending an existing program. • Object - Entities in simulation • An object can represent any entity in solution of problem • An object interacts by sending messages • A computation is characterized in terms of observable behavior of objects

3. 7.2 Object Oriented Thinking • Vocabulary of object-oriented programming • Object : Collection of data and operations • Class: Description of a set of objects; objects with common properties[type of an object] • Subclass: Subset of class, with additional properties; nested class • Superclass: Main class that subclasses fall under • Instance: Technical term for an object of class • Method: Procedure body implementing an operation • Message: Procedure call; request to execute method

4. A class can have inheritance. • Single inheritance: Subclass has one superclass (Java) • Multiple inheritance: Subclass has more than one superclass (C++) • A message can carry parameters. An object will execute a method when gets a message[way responds]. • A class definition specifies the properties of an object. It includes methods it can execute and variables for the object.

5. Object Oriented Thinking (cont.) • When storage is allocated for a variable of the class, we call that an instance.

6. 7.3 Inheritance • Children of a class hierarchy inherit the methods and variables of ancestor methods. • Object determines how it will implement a message. • Information hiding facilitates two kinds of changes: • Implementation change : If all interactions with an object are through it’s interface, then algorithms and data structures are hidden behind the interface can be changed

7. Inheritance change : If all interactions are through the interface to a superclass, then program can be extended by adding a subclass. • Object-oriented programming often done by adopting or extending an existing program. • Subclass is a derived class and superclass is a base class. • A method in a subclass overrides an inherited method of the same name.

8. 7.4 Object-Oriented Programming in C++ • C++ provides a transition path from C to object oriented programming. • Declaring a class: • Constructor: Member function with same name as class; called automatically when lifetime of an object of the class begins[used as an initialization method] • Body of member function can appear within the declaration or separate

9. Three keywords for controlling the visibility of members names. • Public: visible to outside code • Private: not visible to outside code • Keyword: visible through inheritance only • Base and Derived Classes • Derived class: an extension of a base class • Public Base Classes • Identified by the keyword public

10. class <derived> : public <base> { <member-declarations> }; • Members of a public base class retain their visibility in the derived class • Virtual Functions • Allow a derived class to supply the function body. • Are taken from derived class where possible • Initialization and Inheritance • Code for initialization belongs in the constructor for a class

11. Object-Oriented Programming in C++(cont) • Constructor of a base class is called before the derived class • The destructor in derived class is called before the destructor in the base class

12. EXAMPLE • A prime example of object orientation would be a program that draws diagrams. • The object would be shapes. • Can classify a shape based on its properties. • Classification of shape objects • Figure 7.2 Page 257 • Figure 7.3 Page 258

13. Class Shape • Subclass Box • Subclass Line • Subclass Text • Subclass Ellipse • Subclass Circle • Methods of Shape would be • initialize, draw, offset, setwidth, setheight, and setalign

14. Variables of Object Shape • width, height, align • Methods for each class Figure 7.6 Page 261 • Since class box, ellipse, line, text are subclasses of Shape; they will inherit the methods and variables from Shape • What methods and variables would class circle inherit? • Remember that Method Draw from Class box overrides Method Draw from Class Shape

15. Implementation of Shape(s) • Implementation of Class Shape Figure 7.10 page 271

16. 7.5 AN EXTENDED C++ EXAMPLE This section illustrates inheritance in C++ by developing a program to find prime numbers. A prime number is divisible only by itself and 1. Primes: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

17. A Prime Number Sieve • Sieve method is used to compute primes. • The underlying idea is that n is a prime if n is not a multiple of any prime p smaller than n. • Objects used: counter(n) and filter(n)

18. A Base Class class Item { public : Item *source; Item(Item *src) { source = src; } virtual int out() { return 0;} };

19. Derived Classes class Counter : public Item { int value; public : int out() { return value++; } Counter(int v) : Item(0) { value = v; } };

20. Initialization of Derived and Base Classes Counter(int v) : Item(0) { value = v; } • Counter (int v) has an integer argument. • : Item(0) passes the null pointer 0 as an argument to the constructor of the base class. • { value = v; } is the body of the constructor in the derived class.

21. 7.6 DERIVED CLASSES AND INFORMATION HIDING • Inheritance is in terms of an is-a relation on objects. • Hiding inherited members can interfere with a fundamental property, the ability of a derived object to appear wherever a base object is expected. • Example : list and stack

22. Public Base Classes • Syntax for a public base class : class <derived> : public <base> { <member-declarations> }; • Members of a public base class retain their accessibility in the derived class.

23. Public Base Classes • An object of a derived class has an is-a relation with objects of its public base class.

24. Private Base Classes • Syntax for a private base class : class <derived> : private <base> { <member-declarations> }; • A derived class simply shares the code of the private base class. Such code sharing is called implementation inheritance.

25. Private Base Classes • All members derived by <derived> from <base> become private members of <derived>. • Nonprivate inherited members can be made visible by writing their full names in the derived class.

26. Privacy Principle • The private members of a class are accessible only to member functions of the class. • Functions in a derived class cannot access the private members of its base class.

27. class List { cell *rear; public : List(); int empty(); protected : void add(int); void push(int); int get(); }; Privacy Principle

28. Privacy Principle • class Queue : public List { public : Queue(); int get() { return List :: get(); } void put(int x) { add(x) ;} };

29. Privacy Principle A complete listing of the members of class queue. • Public Functions • Queue added constructor function • get added • put added • List :: empty inherited

30. Privacy Principle • Protected functions • add inherited • push inherited • List :: get inherited • Private Variables ( accessible to functions added by Queue ) • none

31. Privacy Principle • Private Variables ( accessible only to inherited functions ) • rear inherited

32. 7.7 OBJECTS IN SMALLTALK Smalltalk is built on the concepts of objects and messages. • Everything is an object. • Data is private to an object. • An object has a notion of self.

33. System classes • Numbers, data structures, and input/output are provided by built-in system classes. • Superclass and subclass vs. base class and derived class

34. Elements of a Class Definition • Instance is a technical term for an object of a class • Variables and methods • Class methods are used primarily to create instances. • Class variables are used to share information between instances.

35. A View of Class Stack in Smalltalk class Stack superclass Object instance variables contents class methods new ^ super new initialize

36. A View of Class Stack in Smalltalk instance methods push: anElement contents addLast: anElement pop ^ contents removeLast initialize contents := OrderedCollection new

37. Instance Variables and Privacy • An instance variable belongs to an instance. Its value can be changed only by operations belonging to the instance. • How are the private variables initialized? • Class variables are shared by all instances of a class. • Global variables are shared by all instances of all classes.

38. Syntax of Messages • Unary messages contents size • Keyword messages aStack push: 54 • Binary messages operators: +, -

39. Expression Evaluation • Evaluation proceeds from left to right. • Unary messages --- highest precedence • Binary messages --- all with the same precedence • Keyword messages --- lowest precedence

40. Expression Evaluation • Examples • contents size = 0 ( contents size ) = 0 • ( ( w*w ) + ( h*h ) ) sqrt w*w + h*h = ( ( w*w ) + h ) * h )

41. Expression Evaluation • The assignment symbol is <-- or := . • A sequence of expressions is separated by dots or periods. w := width / 2 . h := height / 2 . r := ( ( w*w ) + ( h*h ) ) sqrt

42. Returning Values • Return value operator : ^ • Return value operator has lower precedence than other messages. • isEmpty ^ contents size = 0 ^ ( ( contents size ) = 0 )

43. Conditionals and Blocks • An expression sequence enclosed within square brackets, [ and ], is called a block. • x > y if True: [ max := x ] if False: [ max := y ] • Blocks are objects.

44. 7.8 SMALLTALK OBJECTS HAVE A SELF The inheritance rules for Smalltalk • Single inheritance means that each class has at most one superclass. The root of hierarchy is class object . • A subclass inherits variables and methods from its superclass. • A subclass can declare fresh variable names, different from the inherited variables.

45. Methods in the subclass override inherited methods. The rules for methods ensure that an object of a subclass can respond to all the messages that an object of a superclass can.

46. Messages to self • Classes and objects can invoke one of their own methods by sending a message to the special name self. • pop Self isEmpty if True: [ self error : ‘stack empty’ ] if False: [ ^ contents removeLast ]

47. Messages to super • When a subclass overrides an inherited method, the special name super allows the overridden method to be used. • new ^ super new initialize