Classes in VB'Net

1
Classes in VB.Net

• ITE285
• R. Johnson
• DRAFT

2
Classes in Java

• Objects tightly bundle data related
  functions (methods) as members
• This is Encapsulation
• Class is the custom data type from which an
  instance, an object, is declared.
• Inheritance Polymorphism also in VB.NET

3
Parts of An Object in Java

• Data members (private)
• Sets and gets (called explicitly)
• Methods
• Constructor default and overloaded
• Destructor

4
Parts of An Object in VB.NET

• Data members (private)
• Properties (easier sets and gets)
• Methods
• Constructor default and overloaded
• Destructor (when dynamic memory allocated)

5
Property Procedures

• A property is a wrapper around a private data
  member.
• Incorporates sets and gets.
• Feels like accessing a public data member.
• Right side of ? automatically calls get
• Left side of ? automatically calls set

6
Three Ways to Approach Objects

• Classes strengthen separation of Non-Interface
  code
• Think of objects as smart data in collection
• Interface code --- properties for custom dialogs

7
A Simple Class EPAtestCar

• Model
• Miles
• Gallons Gas Used
• Low Emissions Vehicle?
• Method CalcMPG

8
Additions to EPAtestCar

• Property Procedures (streamlined sets gets)
• Default Overloaded Constructors (New)
• Override inherited ToString method
• CompareTo method and interface IComparable (lets
  Array.Sort() work)
• Throwing exceptions from inside a class to be
  caught elsewhere. From property sets?

9
A Common Class Design Pitfall

• Normally do NOT create data members which store
  the results of calls to the classs methods.
• Only store essential, independent data as data
  members.
• Derived or dependent data should be obtained by
  calling the method(s) which compute it, as
  needed.
• Example a class Rectangle with properties for
  length and a method to calculate the area

10
Bad Practice!

• Dim r As Rectangle has an area data member
• r.length 4 set properties
• r.width 2
• r.calcArea() call subroutine method
• that only stores
  result internally
• Messagebox.show (r.area) access property
• storing the
  result of 8

11
Why is this Bad?

• Dim r As Rectangle has an area data member
• r.length 4 set properties
• r.width 2
• r.calcArea() call subroutine method,
  stores area
• r.Length 5 change length!!!
• object now inconsistent, stored area is invalid
  now!
• Messagebox.show (r.area) has 8, should be 10!

12
Good Practice

• Dim r As Rectangleno data member for area!
• r.length 4 set properties
• r.width 2
• call function method that returns a freshly
• computed value as needed
• Messagebox.show (r.calcArea())
• Alternative this could instead be a ReadOnly
  property whose Get does the calculation whenever
  referenced