Object oriented programming

Object Based Programming May 2015

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

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

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

Encapsulation Sometimes referred to as data hiding;an object can expose only those data elements and procedures that it wishes

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

Deriving Subclasses • In VB.NET, we use the reserved word Inherits to establish an inheritance relationship 8

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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.

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.

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

Object oriented programming

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