Object Oriented Programming Chapter 7 Programming Languages by Ravi Sethi

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Object Oriented ProgrammingChapter 7Programming
Languagesby Ravi Sethi
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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

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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 propertiestype 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

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• 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 messageway
  responds.
• A class definition specifies the properties of an
  object. It includes methods it can execute and
  variables for the object.

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Object Oriented Thinking (cont.)

• When storage is allocated for a variable of the
  class, we call that an instance.

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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 its interface, then
  algorithms and data structures are hidden behind
  the interface can be changed

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• 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.

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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 beginsused as an
  initialization method
• Body of member function can appear within the
  declaration or separate

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• 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
  

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• class ltderivedgt public ltbasegt
• ltmember-declarationsgt
• 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

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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

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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

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

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• 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

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Implementation of Shape(s)

• Implementation of Class Shape Figure 7.10 page 271

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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

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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)

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A Base Class

• class Item
• public
• Item source
• Item(Item src) source src
• virtual int out() return 0

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Derived Classes

• class Counter public Item
• int value
• public
• int out() return value
• Counter(int v) Item(0) value v

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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.

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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

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Public Base Classes

• Syntax for a public base class
• class ltderivedgt public ltbasegt
• ltmember-declarationsgt
• Members of a public base class retain their
  accessibility in the derived class.

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Public Base Classes

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

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Private Base Classes

• Syntax for a private base class
• class ltderivedgt private ltbasegt
• ltmember-declarationsgt
• A derived class simply shares the code of the
  private base class. Such code sharing is called
  implementation inheritance.

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Private Base Classes

• All members derived by ltderivedgt from ltbasegt
  become private members of ltderivedgt.
• Nonprivate inherited members can be made visible
  by writing their full names in the derived class.

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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.

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Privacy Principle

• class List
• cell rear
• public
• List()
• int empty()

• protected
• void add(int)
• void push(int)
• int get()

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Privacy Principle

• class Queue public List
• public
• Queue()
• int get() return List get()
• void put(int x) add(x)

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Privacy Principle

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

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Privacy Principle

• Protected functions
• add inherited
• push inherited
• List get inherited
• Private Variables ( accessible to functions added
  by Queue )
• none

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Privacy Principle

• Private Variables ( accessible only to inherited
  functions )
• rear inherited

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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.

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System classes

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

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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.

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A View of Class Stack in Smalltalk

• class Stack superclass Object
• instance variables
• contents
• class methods
• new
• super new initialize

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A View of Class Stack in Smalltalk

• instance methods
• push anElement
• contents addLast anElement
• pop
• contents removeLast
• initialize
• contents OrderedCollection new

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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.

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Syntax of Messages

• Unary messages
• contents size
• Keyword messages
• aStack push 54
• Binary messages
• operators , -

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Expression Evaluation

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

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Expression Evaluation

• Examples
• contents size 0
• ( contents size ) 0
• ( ( ww ) ( hh ) ) sqrt
• ww hh ( ( ww ) h ) h )

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Expression Evaluation

• The assignment symbol is lt-- or .
• A sequence of expressions is separated by dots or
  periods.
• w width / 2 .
• h height / 2 .
• r ( ( ww ) ( hh ) ) sqrt

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Returning Values

• Return value operator
• Return value operator has lower precedence than
  other messages.
• isEmpty
• contents size 0
• ( ( contents size ) 0 )

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Conditionals and Blocks

• An expression sequence enclosed within square
  brackets, and , is called a block.
• x gt y if True max x
• if False max y
• Blocks are objects.

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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.

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• 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.

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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

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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