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INTRODUCTION TO VISUAL BASIC .NET Appendix B 849
Visual Basic .NET enables you to detect and remove errors at run time by supporting structured exception handling. In Visual Basic .NET, you can use Try…Catch…Finally statements to create exception handlers. By using Try…Catch…Finally statements, you can create strong and efficient exception handlers to improve the performance of the application.
You have considered the new and added features of Visual Basic .NET. The following sections discuss the features of an object-oriented programming language.
Features of an Object-Oriented
Programming Language
In an object-oriented programming language, objects serve as the building blocks of a programming language, displaying a unique identity and behavior. A chair, a table, and a book are examples of objects that are used every day. An object in a programming language is defined as an instance of a class. Applications created in an object-oriented programming language are made up of objects.
An object is qualified as an object-oriented programming language if the following features are supported:
Abstraction
Encapsulation
Inheritance
Polymorphism
The next sections will consider each of the features mentioned here in detail.
Abstraction
Before you buy a television set, you consider its size, durability, and features. As a buyer, you may not be interested in knowing about the machinery of the television set.The main features of the television set are more likely to be your primary concern. This is known as abstraction. In a programming language, abstraction helps you focus mainly on the essential aspects of an object. The nonessential aspects are normally overlooked.
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Visual Basic .NET, like any other programming language, provides abstraction through classes and objects. Attributes and behavior shared by similar objects are defined as class. The instance of the class is an object. Each object consists of characteristics and attributes that are the properties of the object. In addition, a set of actions can be performed by each object. The actions that are performed are known as methods. In Visual Basic .NET, you can specify the various properties and methods that are used for objects while creating classes. Abstraction is mainly used to reduce the complexity of an object by exposing only the essential features and methods of an object. Additionally, abstraction helps you generalize an object as a data type. By declaring classes, you can also generalize objects as data types.
Encapsulation
Information hiding or encapsulation means that the nonessential details of an object are hidden. Consider an example: When you switch your television on, it starts functioning. Needless to say that the internal functioning process remains hidden. In other words, the functioning of the television is hidden or encapsulated.
The method of implementing abstraction is encapsulation. As mentioned earlier, abstraction refers mainly to concentrating on the necessary and essential details of an object while ignoring the unnecessary and nonessential ones. Encapsulation achieves this.
The internal implementation of the classes is hidden from the user by encapsulation.Therefore, encapsulation is displaying only the properties and methods of an object. It helps the developers in hiding the complexity of an object and also uses different implementations of the same object.
Inheritance
The earlier versions of Visual Basic supported interface inheritance but not implementation inheritance. However, Visual Basic .NET supports both implementation inheritance and interface inheritance.
Implementation inheritance means that a class is derived from an existing class. The derived class is called subclass, and the class from which it is derived is called base class.
INTRODUCTION TO VISUAL BASIC .NET Appendix B 851
NOTE
The classes that are created in Visual Basic .NET are derived from the Object class, which is a part of the System namespace.
The properties and methods of the base class are inherited by the subclass. In addition, methods and properties can be added to the subclass in order to extend the functionality of the base class. In the derived class, the methods of the base class can also be overridden.
Inheritance also helps you create hierarchies of objects. For example, you can consider a class named animals. The cats class is derived from the animals class, and the lions class is derived from the cats class.
In the preceding example, the class lions inherits the properties and methods of the class cats, which in turn inherits all the properties and methods of the class animals. Therefore, all the properties and methods of the lions class and the cats class are inherited by the animals class.
All the classes that are created in Visual Basic .NET can be inherited by default. Inheritance helps you create complex objects from simpler ones and reuse the code. After a class is created in Visual Basic .NET, it can also be used as a base class in order to create a derived class.
Polymorphism
The ability of an object to exist in different forms is known as polymorphism. Consider an example to have a proper understanding of the term.
If you decide to buy a television set, you either contact a dealer or call the manufacturing company. If you contact a dealer, the dealer first takes the order and then contacts the company. However, if you contact the company directly, the company contacts the dealers of your region and makes the necessary arrangements to deliver the television set. In this case, the dealer and the company are two different classes.The dealer and the company respond differently to the same order. In object-oriented programming, this is known as polymorphism.
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Table B-1 Data Types in Visual Basic .NET |
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Data Type |
Description |
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Integer |
The numeric data is stored.This data type stores the Integer data as a 32-bit |
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(4 bytes) number. |
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Long |
The numeric data that can exceed the range supported by the Integer data type |
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that is stored. It stores the value of Long as a 64-bit (8 bytes) number. |
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Short |
The smaller range of numeric data (between –32,678 to 32,767) is stored.This |
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data type stores the Short data as a 16-bit (2 bytes) number. |
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Byte |
The binary data is stored.This data type can also store ASCII character values |
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in the numeric form. |
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Char |
A single character is stored.This data type stores the Char data as a 16-bit (2 |
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bytes) unsigned number. |
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DateTime |
The date and time data is stored.This data type stores the date and time data |
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as IEEE 64-bit (8 bytes) long integers. |
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String |
The alphanumeric data, which is data containing numbers and text,is stored. |
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Object |
The data of any t ype, such as Integer, Boolean, String, or Long, is stored. |
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Double |
The large floating-point numbers are stored.This data type stores the Double |
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data as an IEEE 64-bit (8 bytes) floating-point number. |
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Single |
The single precision floating-point values are stored.This data type stores the |
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Single data as an IEEE 32-bit (4 bytes) floating-point number. |
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Decimal |
The very large floating-point values are stored.This data type stores the |
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Decimal data as a 128-bit (16 bytes) signed integer to the power of 10. |
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Boolean |
The data that can have only two values is stored.This data type stores the True |
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and False Boolean data as a 16-bit (2 bytes) number. |
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As compared to the earlier versions of Visual Basic, some changes in data types of Visual Basic .NET are mentioned as follows.
The Variant data type is used to store any type of data in Visual Basic 6.0. This is similar to the Object data type in Visual Basic .NET.
The Double data type is used to store a date in Visual Basic 6.0. The DateTime data type stores data in the date and time format in Visual Basic .NET.
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The Currency data type is not supported by Visual Basic .NET. Instead, the Decimal data type is used to store currency values.
After having a look at the various data types, you can now examine how variables are declared in Visual Basic .NET.
Variable Declarations
To provide information about a variable to a program in advance is known as declaring a variable. The Dim statement is used to declare a variable. To declare a variable, you can use the following syntax:
Dim VariableName As type
The As type clause in the Dim statement is optional, and it defines the object type or the data type of the variable that you are declaring. Now, consider the following statement:
Dim int1 as Integer
Dim str1 as String
An Integer variable known as int1 is declared by the first statement, and a String variable known as str1 is declared by the second variable.
Variables can also be declared using the identifier type characters. These characters also specify the data type of a variable. You can consider the following statement as an example:
Dim str1$
In the statement, the identifier type character for a String variable is specified by $. The various identifier type characters that can be used in Visual Basic .NET are listed in Table B-2.
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Table B-2 Identifier Type Characters in Visual Basic .NET |
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Data Type |
Identifier Type Character |
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Integer |
% |
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Long |
& |
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Decimal |
@ |
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String |
$ |
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Single |
! |
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Double |
# |
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You should consider some of the ground rules for naming a variable before discussing the various variable declarations that are possible in Visual Basic .NET. However, it is not necessary for you to follow these naming conventions. Following the naming convention makes the code easy to understand for anyone who wants to understand the code.
Some of the ground rules of naming a variable are:
A variable must begin with a letter.
A variable cannot contain a period or identifier type character.
A variable must not exceed 255 characters.
A variable must be unique within the same scope, defined as the range from which a variable can be accessed, such as a procedure, a form, or a module.
NOTE
A module is defined as a collection of procedures where a procedure is a set of statements used to perform some specific tasks.
You learned how to declare a variable. You will now learn how to initialize variables.
INTRODUCTION TO VISUAL BASIC .NET Appendix B 857
The public variables can be used across modules and also can be declared at the module-level. A public variable is declared in the following statements.
Public Dim int1 As Integer
or
Public int1 As Integer
Constants
Suppose you need to use a particular value in an application. The application needs to calculate and display the percentage of marks obtained by each student in an examination. To calculate the percentage of marks, the application needs to use the maximum score at a number of places. In this case, instead of repeating each value every time, you can use constants. A variable whose value remains the same during the execution of a program is called a constant.
To declare a constant, you can use the following statement:
Const maxMarks As Integer = 100
or
Const maxMarks = 100
Each of the previously mentioned statements declares a constant by the name maxMaks and initializes it with the value 100. These statements use a const keyword to declare a constant.
In case of any change in value, the processing of constants is faster than with variables; only the value at the point of declaring the constant needs to be changed.
You have learned about the variables and related concepts. You will now learn how to perform various operations on these variables.
Operators
A unit of code that performs an operation on one or more variables or elements is known as an operator. An operator can be used to perform various operations, such as arithmetic operations, concatenation operations, comparison operations, and logical operations.