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Classes and Class Libraries Examples and Hints. November 9, 2010. Complex Number Class. A CLASSic example of a Class. Demonstrates a good use of ToString . Demonstrates operator overloading. Operator overloading. Allows you to define common operators for your custom classes.

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Classes and class libraries examples and hints

Classes and Class LibrariesExamples and Hints

November 9, 2010


Complex number class
Complex Number Class

  • A CLASSic example of a Class.

  • Demonstrates a good use of ToString.

  • Demonstrates operator overloading.


Operator overloading
Operator overloading

  • Allows you to define common operators for your custom classes.

    • What does it mean for two objects of your class to be equal?

    • How do you add two objects? Multiply?

    • How do you convert an object of your class to another type?



Class complex the basics
Class Complex: The Basics

  • I got this code from the Stevens book.

  • Note that Stevens cheats—he uses Public variables instead of properties for Re and Im.

  • Public variables work just like our standard Public properties, but they do not allow for

  • adding validation code.

  • If you anticipate ever adding on to your class, you should use Public properties with

  • with Private variables, like you did in assignment 3.


Tostring
ToString

  • Complex numbers are generally written as

    a + bi

    where a is the real part and b the imaginary part.

  • The Complex class incorporates this format into its ToString function:


Operator overloads
Operator Overloads

  • You can add operator functions to your classes, such as Equals, Plus, Minus, Times, etc.

  • However, VB allows you to define what the standard operator symbols (=, +, -, *, etc.) mean for your class.

  • This makes your code easier to read AND write.


Equals
Equals

  • Being able to test equality is frequently important.

  • In your custom classes, you can define what “=“ means.

  • Note that if you define “=“, you must also define “<>”.

  • Note also that “>” and “<“ are not defined, since they are not well defined for complex numbers.

  • All operator overloads must be declared Public Shared.

  • “Shared” means that the operator belongs to the class as a whole, not to individual objects.

  • The equals that is being overloaded here is the comparison equals, the one that comes after “If”, not the assignment equals.

  • Remember that for objects, the assignment equals works on references (addresses), not the objects themselves.



Conversions
Conversions

  • VB allows you to define conversions from your class by overloading CType.

  • The code below effectively defines CDbl for the Complex class as being the absolute value:


Absolute value as readonly property
Absolute Value as ReadOnly Property

  • Personally, I would rather see absolute value implemented as a property, not a conversion.

  • The property is ReadOnly since it is not something that you could assign directly; it depends on the real and imaginary parts of the number.

  • For example, the complex numbers 1, i, -1, and –i all have the same absolute value.


Building on the complex class
Building on the Complex class

  • Can you think of other properties, functions, operators, constructors, conversions, etc. that could be added to the Complex class?

  • Some classes that might be similar to Complex?

    • Vector

    • Matrix

    • Field (electric, gravitational)


Wrapper classes
Wrapper Classes

  • VB’s .NET framework contains lots of wrapper classes.

  • A wrapper class encapsulates some difficult or tedious code which performs something useful into a simple interface.

  • For example, it would take a lot of code to write your own web browser into your program.

  • Fortunately, Microsoft has provided the WebBrowser control (in the ToolBox).

  • With the WebBrowser control (a class, of course), you get pretty much all of Internet Explorer’s functionality with just a few lines of code.


Webbrowser control
WebBrowser Control

  • The VB Toolbox offers a WebBrowser control.

  • This is basically a customizable version of Internet Explorer that you can build into your programs.

  • Possible uses:

    • Display pictures from the web (as an alternative to the PictureBox control)

    • Display help files

    • Allow users of your program to link to specific web pages, while preventing access to other pages (or general browsing)


Webbrowser demo
WebBrowser Demo

  • Open the “HTML Lecture” VB project.

  • Click on WebBrowser Demo.

  • This form will appear:


The webbrowser form
The WebBrowser Form

  • frmBrowser contains a SplitContainer control.

  • In the top half is a WebBrowser control.

  • In the bottom is a TextBox.

  • When the WebBrowser navigates to a new web page, the TextBox displays the HTML code for that page.




Navigation code
Navigation Code

  • The WebBrowser control’s “AllowNavigation” property determines if the user can use the browser as a true web browser or not.

  • If set to False, they will not be able to connect to linked pages by clicking on hyperlinks.

  • Set AllowNavigation to False if you want to restrict the user to web pages related to your program.


Getting the html source code
Getting the HTML source code

  • If you want to retrieve the source code (usually HTML or XML) from a web page, use the WebBrowser’sDocumentText property.

  • The above code demonstrates this.

  • The code is placed in the WebBrowser’s Navigated event; that is, it waits until the new page is completely loaded.

  • Once you have retrieved the DocumentText, you can search for keywords or values, such as a stock price or sports score, using the various String functions.


Dbconn another wrapper class
DbConn: Another Wrapper Class

  • Most VB textbooks show you how to connect to a database and retrieve data, typically using code like this:


Dbconn
DbConn

  • This code is tedious, difficult to understand or remember.

  • As written, it works only with Access 2007 databases (Microsoft.ACE.OLEDB.12.0).

  • The function encapsulates the code to some degree, but we can get greater functionality and flexibility by incorporating it into a class.

  • I did this about three years ago, both for my work at UMTRI and for this course.


Dbconn public interface
DbConn: Public Interface

  • Public Sub New(ByValFileName As String)

  • Public Sub New(ByValServerName As String, ByValDatabaseName As String)

    • These two constructors identify the database to be connected to, and test the connection. If the database cannot be located or it can’t be open, the constructor raises an exception.

    • The first constructor (one parameter) is for Access databases;

    • The second (overloaded) constructor (two parameters) is for SQL Server databases.


Opendatabaseconnection closedatabaseconnection
OpenDatabaseConnection, CloseDatabaseConnection

  • Public Sub OpenDatabaseConnection()

  • Public Sub CloseDatabaseConnection()

    • There are time and availability costs to opening a database and leaving it open.

    • In general, if you need to run a lot of queries in a row, you should open the connection once, run all the queries, and then close the connection.

    • I didn’t show you these subs for assignment 2, but you may want to use them if you find your data-connected program running slowly.

    • Both GetDataTable and ExecuteSQL leave the connection in the condition that they found it (which defaults to closed).


Running the queries
Running the Queries

  • You know how to use the remaining parts of DbConn’s user interface:

  • Public Function GetDataTable(ByValsql As String) As DataTable

  • Public Sub ExecuteSQL(ByValsql As String)


  • Connection encapsulated
    Connection Encapsulated

    • Together, the public interface of DbConn encapsulates all of the complex code we saw earlier.

    • That interface is,again:

      • Public Sub New(ByValFileName As String)

      • Public Sub New(ByValServerName As String, ByValFileName As String)

      • Public Sub OpenDatabaseConnection()

      • Public Sub CloseDatabaseConnection()

      • Public Function GetDataTable(ByValsql As String) As DataTable

      • Public Sub ExecuteSQL(ByValsql As String)


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