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Object oriented programming

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Object oriented programming

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Object oriented programming

  1. 1. Object Based Programming May 2015
  2. 2. Summary Slide • Instantiating An Object • Encapsulation • Inheritance • Polymorphism ▫ Overriding Methods ▫ Overloading vs. Overriding • Implementing a Time Abstract Data type with a Class ▫ Controlling Access to Members ▫ Initializing Class Objects: Constructors ▫ Properties ▫ Composition ▫ Shared Class Members ▫ Const and ReadOnly Members • Garbage Collection
  3. 3. Object Terminology Review • Object - like a noun, a thing ▫ Buttons, Text Boxes, Labels • Properties - like an adjective, characteristics of object ▫ Text, ForeColor, Checked, Visible, Enabled • Methods - like a verb, an action or behavior, something the object can do or have done to it ▫ ShowDialog, Focus, Clear, ToUpper, ToLower • Events - object response to user action or other events ▫ Click, Enter, Activate
  4. 4. Instantiating An Object • Creating a new object based on a class • Create an instance of the class by using the New keyword and specify the class
  5. 5. Encapsulation Sometimes referred to as data hiding;an object can expose only those data elements and procedures that it wishes
  6. 6. Inheritance • Inheritance is a form of reusability in which classes are created by absorbing an existing class’s data and behaviors and improving by adding new capabilities. • Inheritance allows a software developer to derive a new class from an existing one • The existing class is called the parent class, or superclass, or base class • The derived class is called the child class or subclass. • As the name implies, the child inherits characteristics of the parent 6
  7. 7. Inheritance 7
  8. 8. Deriving Subclasses • In VB.NET, we use the reserved word Inherits to establish an inheritance relationship 8
  9. 9. Controlling Inheritance • Visibility modifiers determine which class members get inherited and which do not • Variables and methods declared with public visibility are inherited, and those with private visibility are not • But public variables violate our goal of encapsulation • There are two more visibility modifiers that helps in inheritance situations: Protected and Friend 9
  10. 10. The Protected Modifier • The Protected visibility modifier allows a member of a base class to be inherited into the child • But Protected visibility provides more encapsulation than public does • However, Protected visibility is not as tightly encapsulated as Private visibility 10
  11. 11. The Friend Modifier • The Friend visibility modifier allows a member of a base class to be inherited into the child only if the the derived class is in the same assembly 11
  12. 12. The MyBase Reference 12
  13. 13. Single vs. Multiple Inheritance • VB.NET supports single inheritance, meaning that a derived class can have only one parent class • Multiple inheritance allows a class to be derived from two or more classes, inheriting the members of all parents • Collisions, such as the same variable name in two parents, have to be resolved • In most cases, the use of interfaces gives us the best aspects of multiple inheritance without the overhead
  14. 14. Polymorphism • Different classes of objects may have behaviors that are named the same but are implemented differently • Programmers can request an action without knowing exactly what kind of object they have or exactly how it will carry out the action
  15. 15. Polymorphism Implemented • Overloading ▫ Argument type determines which version of a method is used ▫ Example: MessageBox.Show method • Overriding ▫ Refers to a class that has the same method name as its base class ▫ Method in subclass takes precedence
  16. 16. Overriding Methods • A child class can override the definition of an inherited method in favor of its own • That is, a child can redefine a method that it inherits from its parent • The new method must have the same signature as the parent's method, but can have different code in the body 16
  17. 17. Overloading vs. Overriding • Don't confuse the concepts of overloading and overriding • Overloading deals with multiple methods in the same class with the same name but different signatures • Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature 17
  18. 18. Reusability • The main purpose behind OOOP and Inheritance in particular • New classes created with Class Module can be used in multiple projects • Each object created from the class can have its own properties
  19. 19. Multitier Applications • Common use of classes is to create multitier applications • Each of the functions of a multitier application can be coded in a separate component and stored and run on different machines • Goal is to create components that can be combined and replaced
  20. 20. Three-tier Model • Most common implementation of multitier Presentation Tier Business Tier Data Tier User Interface Forms Controls Menus Business Objects Validation Calculations Business Logic Business Rates Data Retrieval Data Storage
  21. 21. Implementing a Time Abstract Data type with a Class • VB programmers concentrate on creating their own user-defined types called classes (also referred as programmer defined types) • Classes in VB facilitate the creation of special data types, called abstract data types (ADT)
  22. 22. Class Scope • A class’s instance variables and methods belong to the class’s scope. • Class members that are visible can be accessed only through a “handle” (ObjectReferenceName.memberName) • Variables within methods ▫ Only methods can access that variable • Keyword Me is a hidden instance variable can be accessed in a method by preceding its name with the keyword Me and dot operator
  23. 23. Controlling Access to Members • The member access modifiers Public, Private, Protected, and Friend control access to a class’s instance variables and methods. • Control access to a class’s instance variables and methods ▫ Public: Serves primarily to present interfaces of a class ▫ Private: Holds clients private data safely ▫ Get and set functions have ability to access private data
  24. 24. Initializing Class Objects: Constructors • A constructor method initializes its class’s members • When appropriate, provide a default constructor to ensure that every object is initialized with meaningful values • Parametized constructors have arguments • If a class does not have a defined constructor, the compiler will create an empty constructor. • If an instance variable is not initialized the compiler will assign a default value • Overloaded Constructors must have different numbers and/or types and/or orders of parameters
  25. 25. Properties • Methods in a class can manipulate the class’s Private instance variables. Public methods allow other object to change a class’s properties. • Get accessor ▫ In Visual Basic instance variables as private does not guarantee data integrity • Set accessor ▫ Cannot return values indicating a failed attempt to assign invalid data to objects of the class ▫ Control the setting of instance variables to valid values • Get and Set accessors are not required • A property with only Get accessor is called ReadOnly • A property with only Set accessor is called WriteOnly • After we define a property, we can use it in the same way as we use a variable.
  26. 26. Using the Me Reference
  27. 27. Shared Class Members • Contains only one copy of this variable in memory • When a single copy of the data will suffice, use Shared class variables to save storage. • Shared class variables are not the same as global variables because Shared class variables have class scope • A class’s shared class members are available as soon as the class is loaded into memory at execution time • Shared method has no Me reference • A shared method cannot access non-shared class members.
  28. 28. Const and ReadOnly Members
  29. 29. Garbage Collection • Resource leaks ▫ Objects must have an efficient way to return memory and release resources when the program no longer uses those objects • Memory leaks ▫ In Visual Basic memory is reclaimed automatically, hence it experiences rare memory leaks as compared to C and C++ • Finalization ▫ Finalizer method performs termination housekeeping on that object just before the garbage collector reclaims the object's memory. • Feature of .NET Common Language Runtime (CLR) that cleans up unused components • Periodically checks for unreferenced objects and releases all memory and system resources used by the objects • Microsoft recommends depending on Garbage Collection rather than Finalize procedures

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