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Abstraction (VB.Net and Java Cross Reference Guide)

By Mike Prestwood

VB.Net versus Java: A side by side comparison between VB.Net and Java.

 
OOP Details
 

More object oriented (OO) stuff.

Abstraction

[Other Languages] 

General Info: Abstract Class / Abstract Member

An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.

Languages Focus

Abstraction is supported at various levels with each language. A language could enforce abstraction at the class level (either enforcing a no-instantiation rule or a only abstract members rule), and with class members (member methods and/or properties).

VB.Net:   MustInherit, MustOverride, Overrides

VB.Net supports abstract class members and abstract classes using the MustInherit and MustOverride modifiers.

An abstract class is indicated with a MustInherit modifier and is a class with one or more abstract members and you cannot instantiate an abstract class. However, you can have additional implemented methods and properties.

An abstract member is either a method (implicitly virtual), property, indexer, or event in an abstract class. You can add abstract members ONLY to abstract classes using the MustOverride keyword. Then you override it in a descendant class with Overrides.

Syntax Example:
Public MustInherit Class Cyborg
  Public MustOverride Sub Speak(ByVal pMessage As String)
End Class

Public Class Series600
  Inherits Cyborg

  Public Overrides Sub Speak(ByVal pMessage As String)
    MessageBox.Show(pMessage)
  End Sub
End Class
Java:   abstract

Java supports marking a full class as abstract as well as class members. A subclass must either implement the abstract members or you must declare the subclass abstract (which delays the implementation to it's subclass).

Syntax Example:
public abstract class Dog {
  abstract void Bark();
}












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