Classes and Class Libraries Examples and Hints

1. Classes and Class LibrariesExamples and Hints November 9, 2010

2. Complex Number Class • A CLASSic example of a Class. • Demonstrates a good use of ToString. • Demonstrates operator overloading.

3. 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?

4. Show the Program

5. 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.

6. 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:

7. 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.

8. 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.

9. Do the Math

10. 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:

11. 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.

12. 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)

13. 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.

14. 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)

15. WebBrowser Demo • Open the “HTML Lecture” VB project. • Click on WebBrowser Demo. • This form will appear:

16. 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.

17. Open New Browser Code

18. Go To Google Code

19. 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.

20. 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.

21. DbConn: Another Wrapper Class • Most VB textbooks show you how to connect to a database and retrieve data, typically using code like this:

22. 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.

23. 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.

24. 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).

25. 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)

26. 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)