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Constructors. Constructors. You got plenty of experience using constructors in the Marching Band program. A constructor is the subroutine which creates objects from a class. Every class has a default constructor which takes no parameters– New().

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Presentation Transcript
constructors1
Constructors
  • You got plenty of experience using constructors in the Marching Band program.
  • A constructor is the subroutine which creates objects from a class.
  • Every class has a default constructor which takes no parameters– New().
  • This default constructor is there even if you don’t see it.
threedpoint
ThreeDPoint
  • In the class definition, there is no Sub New.
  • However, I can still call the default constructor like this:
creating a constructor
Creating a Constructor
  • However, we can create our own constructor.
  • There are two main reasons for creating your own constructor:
    • To initialize the value of variables using parameters; and
    • To create any objects that the current object will need; that is, calling constructors of other classes. Here’s the Rank class’s constructor from Marching Band:
  • Notice that it
    • Initializes the mStepSize variable;
    • Creates the 10 members of the rank by calling BandMember’s constructor.
creating a constructor1
Creating a Constructor
  • Here’s a simple constructor for the ThreeDPoint class. It simply initializes the variables.
constructors in action
Constructors in Action
  • To understand how to declare variables and create objects using constructors,
  • View the ConstructorsInAction video.
overloads
Overloads
  • VB allows functions and subs (including constructors) to be overloaded.
  • This means that you can have two or more procedures with the same name.
  • They just have to differ in the number and/or type of parameters.
  • Overloads are used widely in the built-in VB.NET classes (you’ll see many in the graphics routines), but you can create your own overloads as well.
tostring
ToString
  • Older versions of VB, including Visual Basic for Applications in Excel, do not have a “ToString” function.
  • VBA is a loosely-typed language that does all sorts of datatype conversions in the background for you (sometimes well, sometimes not).
  • VB.NET, the descendant of VBA (VB 6), is a strongly typed language, which means that functions, subs and properties can require a particular parameter type.
  • If a String parameter type is required you can put ToString on the end of a variable or expression to convert it to a string.
tostring1
ToString
  • For example: Notice that when I try to put a numeric value into a Text property, I get an error:
  • ToString is the cure:
  • (Note: I added the extra blank lines so the error message wouldn’t cover up the line above. This doesn’t mean that I now approve of lots of blank lines!)
tostring2
ToString
  • The standard datatypes (even String) have built-in ToString methods.
  • ToString can even be applies to expressions:
  • However, the classes that you create will have a default ToString method that’s pretty useless: It just displays the name of the class.
  • And this can be annoying when you start adding objects of your own classes to ListBoxes and ComboBoxes!
using tostring to format numbers
Using ToString to Format Numbers
  • ToString can be used to format numbers, like this:
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