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UNIT IV OBJECT-ORIENTATION, CONCURRENCY, AND EVENT HANDLING

UNIT IV OBJECT-ORIENTATION, CONCURRENCY, AND EVENT HANDLING. Object-Oriented Programming. Think from the perspectives of data (“things”) and their interactions with the external world Object Data Method: interface and message Class The need to handle similar “things”

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UNIT IV OBJECT-ORIENTATION, CONCURRENCY, AND EVENT HANDLING

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  1. UNIT IV OBJECT-ORIENTATION, CONCURRENCY, AND EVENT HANDLING IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  2. Object-Oriented Programming • Think from the perspectives of data (“things”) and their interactions with the external world • Object • Data • Method: interface and message • Class • The need to handle similar “things” • American, French, Chinese, Korean  abstraction • Chinese: northerners, southerners  inheritance • Dynamic binding, polymorphism IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  3. What OOP Allows You? • You analyze the objects with which you are working (attributes and tasks on them) • You pass messages to objects, requesting them to take action • The same message works differently when applied to the various objects • A method can work with different types of data, without the need for separate method names • Objects can inherit traits of previously created objects • Information can be hidden better IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  4. Abstraction • Two types of abstractions: • Process abstraction: subprograms • Data abstraction • Floating-point data type as data abstraction • The programming language will provide (1) a way of creating variables of the floating-point data type, and (2) a set of operators for manipulating variables • Abstract away and hide the information of how the floating-point number is presented and stored • Need to allow programmers to do the same • Allow them to specify the data and the operators IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  5. Abstraction Data Type • Abstract data type: a user-defined data type • Declaration of the type and protocol of operations on objects of the type, i.e., type’s interface, are defined in a syntactic unit; interface indep. of implementation • Representation of objects of the type is hidden from program units that use these objects; only possible operations are those provided in type's definition class data type int object variable i, j, k method operators +, -, *, / y = stack1.top()+3; vs y = (-x) + 3; IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  6. Advantages of Data Abstraction • Advantage of having interface independent of object representation or implementation of operations: • Program organization, modifiability (everything associated with a data structure is together), separate compilation • Advantage of 2nd condition (info. hiding) • Reliability: By hiding data representations, user code cannot directly access objects of the type or depend on the representation, allowing the representation to be changed without affecting user code IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  7. Language Requirements for ADTs • A syntactic unit to encapsulate type definition • A method of making type names and subprogram headers visible to clients, while hiding actual definitions • Some primitive operations that are built into the language processor • Example: an abstract data type for stack • create(stack), destroy(stack), empty(stack), push(stack, element), pop(stack), top(stack) • Stack may be implemented with array, linked list, ... IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  8. Abstract Data Types in C++ • Based on C struct type and Simula 67 classes • The class is the encapsulation device • All of the class instances of a class share a single copy of the member functions • Each instance has own copy of class data members • Instances can be static, stack dynamic, heap dynamic • Information hiding • Private clause for hidden entities • Public clause for interface entities • Protected clause for inheritance (Chapter 12) IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  9. Implicitly inlined  code placed in caller’s code Member Functions Defined in Class class Stack { private: int *stackPtr, maxLen, topPtr; public: Stack() { // a constructor stackPtr = new int [100]; maxLen = 99; topPtr = -1; }; ~Stack () {delete [] stackPtr;}; void push (int num) {…}; void pop () {…}; int top () {…}; int empty () {…}; } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  10. Language Examples: C++ (cont.) • Constructors: • Functions to initialize the data members of instances (they do not create the objects) • May also allocate storage if part of the object is heap-dynamic • Can include parameters to provide parameterization of the objects • Implicitly called when an instance is created • Can be explicitly called • Name is the same as the class name IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  11. Language Examples: C++ (cont.) • Destructors • Functions to clean up after an instance is destroyed; usually just to reclaim heap storage • Implicitly called when the object’s lifetime ends • Can be explicitly called • Name is the class name, preceded by a tilde (~) • Friend functions or classes: to allow access to private members to some unrelated units or functions • Necessary in C++ IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  12. Uses of the Stack Class void main() { int topOne; Stack stk; //create an instance of the Stack class stk.push(42); // c.f., stk += 42 stk.push(17); topOne = stk.top(); // c.f., &stk stk.pop(); ... } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  13. Member Func. Defined Separately // Stack.h - header file for Stack class class Stack { private: int *stackPtr, maxLen, topPtr; public: Stack(); //** A constructor ~Stack(); //** A destructor void push(int); void pop(); int top(); int empty(); } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  14. Member Func. Defined Separately // Stack.cpp - implementation for Stack #include <iostream.h> #include "Stack.h" using std::cout; Stack::Stack() { //** A constructor stackPtr = new int [100]; maxLen = 99; topPtr = -1;} Stack::~Stack() {delete[] stackPtr;}; void Stack::push(int number) { if (topPtr == maxLen) cerr << "Error in push--stack is full\n"; else stackPtr[++topPtr] = number;} ... IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  15. Abstract Data Types in Java • Similar to C++, except: • All user-defined types are classes • All objects are allocated from the heap and accessed through reference variables • Methods must be defined completely in a class an abstract data type in Java is defined and declared in a single syntactic unit • Individual entities in classes have access control modifiers (private or public), rather than clauses • No destructor  implicit garbage collection IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  16. An Example in Java class StackClass { private int [] stackRef; private int maxLen, topIndex; public StackClass() { // a constructor stackRef = new int [100]; maxLen = 99; topPtr = -1;}; public void push (int num) {…}; public void pop () {…}; public int top () {…}; public boolean empty () {…}; } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  17. An Example in Java public class TstStack { public static void main(String[] args) { StackClass myStack = new StackClass(); myStack.push(42); myStack.push(29); System.out.println(“:“+myStack.top()); myStack.pop(); myStack.empty(); } } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  18. “Hello World!” Compared C++ #include <iostream> using namespace std; int main(){ cout<<"Hello World!"<<endl; } C #include <stdio.h> int main(void){ print("Hello world!"); } Ruby puts 'Hello, world!' or class String def say puts self end end 'Hello, world!'.say Java public class HelloWorld { public static void main(String[] args){ System.out.println ("Hello world!"); } } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  19. Parameterized ADTs • Parameterized abstract data types allow designing an ADT that can store any type elements (among other things): only an issue for static typed languages • Also known as generic classes • C++, Ada, Java 5.0, and C# 2005 provide support for parameterized ADTs IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  20. Parameterized ADTs in C++ • Make Stack class generic in stack size by writing parameterized constructor function class Stack { ... Stack (int size) { stk_ptr = new int [size]; max_len = size - 1; top = -1; }; ... } Stack stk(150); IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  21. Parameterized ADTs in C++ (cont.) • Parameterize element type by templated classtemplate <class Type>class Stack { private: Type *stackPtr; int maxLen, topPtr; public: Stack(int size) { stackPtr = new Type[size]; maxLen = size-1; topPtr = -1; } ...Stack<double> stk(150); Instantiated by compiler IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  22. Generalized Encapsulation • Enclosure for an abstract data type defines a SINGLE data type and its operations • How about defining a more generalized encapsulation construct that can define any number of entries/types, any of which can be selectively specified to be visible outside the enclosing unit • Abstract data type is thus a special case IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  23. Encapsulation Constructs • Large programs have two special needs: • Some means of organization, other than simply division into subprograms • Some means of partial compilation (compilation units that are smaller than the whole program) • Obvious solution: a grouping of logically related code and data into a unit that can be separately compiled (compilation units) • Such collections are called encapsulation • Example: libraries IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  24. Means of Encapsulation: Nested Subprograms • Organizing programs by nesting subprogram definitions inside the logically larger subprograms that use them • Nested subprograms are supported in Ada, Fortran 95, Python, and Ruby IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  25. Encapsulation in C • Files containing one or more subprograms can be independently compiled • The interface is placed in a header file • Problem: • The linker does not check types between a header and associated implementation • #include preprocessor specification: • Used to include header files in client programs to reference to compiled version of implementation file, which is linked as libraries IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  26. Encapsulation in C++ • Can define header and code files, similar to those of C • Or, classes can be used for encapsulation • The class header file has only the prototypes of the member functions • The member definitions are defined in a separate file  Separate interface from implementation • Friends provide a way to grant access to private members of a class • Example: vector object multiplied by matrix object IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  27. Friend Functions in C++ class Matrix; class Vector { friend Vector multiply(const Matrix&, const Vector&); ... } class Matrix { friend Vector multiply(const Matrix&, const Vector&); ... } Vector multiply(const Matrix& ml, const Vector& vl) { ... } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  28. Naming Encapsulations • Encapsulation discussed so far is to provide a way to organize programs into logical units for separate compilation • On the other hand, large programs define many global names; need a way to avoid name conflicts in libraries and client programs developed by different programmers • A naming encapsulation is used to create a new scope for names IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  29. Naming Encapsulations (cont.) • C++ namespaces • Can place each library in its own namespace and qualify names used outside with the namespace namespace MyStack { ... // stack declarations } • Can be referenced in three ways: MyStack::topPtr using MyStack::topPtr; p = topPtr; using namespace MyStack; p = topPtr; • C# also includes namespaces IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  30. Naming Encapsulations (cont.) • Java Packages • Packages can contain more than one class definition; classes in a package are partial friends • Clients of a package can use fully qualified name, e.g., myStack.topPtr, or use import declaration, e.g., import myStack.*; • Ada Packages • Packages are defined in hierarchies which correspond to file hierarchies • Visibility from a program unit is gained with the with clause IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  31. Naming Encapsulations (cont.) • Ruby classes are name encapsulations, but Ruby also has modules • Module: • Encapsulate libraries of related constants and methods, whose names in a separate namespace • Unlike classes  cannot be instantiated or subclassed, and they cannot define variables • Methods defined in a module must include the module’s name • Access to the contents of a module is requested with the require method IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  32. Ruby Modules module MyStuff PI = 3.1415 def MyStuff.mymethod1(p1) ... end def MyStuff.mymethod(p2) ... end end Require ‘myStuffMod’ myStuff.mymethod1(x) IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  33. Support for Object-Oriented Programming IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  34. Software Productivity • Pressure on software productivity vs. continual reduction in hardware cost • Productivity increases can come from reuse • Abstract data types (ADTs) for reuse? • Mere ADTs are not enough • ADTs are difficult to reuse—always need changes for new uses, e.g., circle, square, rectangle, ... • All ADTs are independent and at the same level hard to organize program to match problem space • More, in addition to ADTs, are needed IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  35. What Are Needed? • Given a collection of related ADTs, need to factor out their commonality and put it in a new type  abstraction • The collection of ADTs then inherit from the new type and add their own • class Shape { • private: • int x; int y; • public: • Shape(int a, int b); • ... • virtual void draw(); • }; • class Circle: public Shape { • private: • int radius; • public: • Circle(int x1, y2, r1); • ... • void draw(); • }; IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  36. What Are Needed? (cont.) • For classes in an inheritance relationship, a method call may need to be bound to a specific object of one of the classes at run time Shape *shape_list[3]; // array of shape objects shape_list[0] = new Circle; shape_list[1] = new Square; shape_list[2] = new Triangle; for(int i = 0; i < 3; i++){ shape_list[i].draw(); } • Polymorphism and dynamic binding IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  37. Object-Oriented Programming • An object-oriented language must provide supports for three key features: • Abstract data types • Inheritance: the central theme in OOP and languages that support it • Polymorphism and dynamic binding • What does it mean in a pure OO program, where x, y, and 3 are objects? y = x + 3; IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  38. Object-Oriented Concepts • ADTs are usually called classes, e.g. Shape • Class instances are called objects, e.g. shape_list[0] • A class that inherits is a derived class or a subclass, e.g. Circle, Square • The class from which another class inherits is a parent class or superclass, e.g. Shape • Subprograms that define operations on objects are called methods, e.g. draw() IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  39. Object-Oriented Concepts (cont.) • Calls to methods are called messages, e.g. shape_list[i].draw(); • Object that made the method call is the client • The entire collection of methods of an object is called its message protocol or message interface • Messages have two parts--a method name and the destination object • In the simplest case, a class inherits all of the entities of its parent IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  40. Inheritance • Allows new classes defined in terms of existing ones, i.e., by inheriting common parts • Can be complicated by access controls to encapsulated entities • A class can hide entities from its subclasses (private) • A class can hide entities from its clients • A class can also hide entities for its clients while allowing its subclasses to see them • A class can modify an inherited method • The new one overrides the inherited one • The method in the parent is overridden IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  41. Inheritance (cont.) • There are two kinds of variables in a class: • Class variables - one/class • Instance variables - one/object, object state • There are two kinds of methods in a class: • Class methods – accept messages to the class • Instance methods – accept messages to objects • Single vs. multiple inheritance • One disadvantage of inheritance for reuse: • Creates interdependencies among classes that complicate maintenance IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  42. Dynamic Binding • A polymorphic variable can be defined in a class that is able to reference (or point to) objects of the class and objects of any of its descendants Shape *shape_list[3]; // array of shapes shape_list[0] = new Circle; shape_list[1] = new Square; shape_list[2] = new Triangle; for(int i = 0; i < 3; i++){ shape_list[i].draw(); } IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  43. Dynamic Binding • When a class hierarchy includes classes that override methods and such methods are called through a polymorphic variable, the binding to the correct method will be dynamic, e.g.,shape_list[i].draw(); • Allows software systems to be more easily extended during both development and maintenance, e.g., new shape classes are defined later IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  44. Dynamic Binding (cont.) • An abstract method is one that does not include a definition (it only defines a protocol) • An abstract class is one that includes at least one virtual method • An abstract class cannot be instantiated • class Shape { • private: • int x; int y; • public: • ... • virtual void draw(); • }; • class Circle: public Shape { • private: • int radius; • public: • ... • void draw() {...}; • }; IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  45. Design Issues for OOP Languages • The exclusivity of objects • Are subclasses subtypes? • Type checking and polymorphism • Single and multiple inheritance • Object allocation and deallocation • Dynamic and static binding • Nested classes • Initialization of objects IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  46. The Exclusivity of Objects • Option 1: Everything is an object • Advantage: elegance and purity • Disadvantage: slow operations on simple objects • Option 2: Add objects to existing typing system • Advantage: fast operations on simple objects • Disadvantage: confusing typing (2 kinds of entities) • Option 3: Imperative-style typing system for primitives and everything else objects • Advantage: fast operations on simple objects and a relatively small typing system • Disadvantage: still confusing by two type systems IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  47. Are Subclasses Subtypes? • Does an “is-a” relationship hold between a parent class object and an object of subclass? • If a derived class is-a parent class, then objects of derived class behave same as parent class object subtype small_Int is Integer range 0 .. 100; • Every small_Int variable can be used anywhere Integer variables can be used • A derived class is a subtype if methods of subclass that override parent class are type compatible with the overridden parent methods • A call to overriding method can replace any call to overridden method without type errors IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  48. Type Checking and Polymorphism • Polymorphism may require dynamic type checking of parameters and the return value • Dynamic type checking is costly and delays error detection • If overriding methods are restricted to having the same parameter types and return type, the checking can be static IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  49. Single and Multiple Inheritance • Multiple inheritance allows a new class to inherit from two or more classes • Disadvantages of multiple inheritance: • Language and implementation complexity: if class C needs to reference both draw() methods in parents A and B, how to do? If A and B in turn inherit from Z, which version of Z entry in A or B should be ref.? • Potential inefficiency: dynamic binding costs more with multiple inheritance (but not much) • Advantage: • Sometimes it is quite convenient and valuable IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

  50. Object Allocation and Deallocation • From where are objects allocated? • If behave like ADTs, can be allocated from anywhere • Allocated from the run-time stack • Explicitly created on the heap (via new) • If they are all heap-dynamic, references can be uniform thru a pointer or reference variable • Simplifies assignment: dereferencing can be implicit • If objects are stack dynamic, assignment of subclass B’s object to superclass A’s object is value copy, but what if B is larger in space? • Is deallocation explicit or implicit? IFETCE/ME CSE/I YEAR/II SEM/CP7203/PPL/UNIT 4/PPT/VER 1.0

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