Abstract Classes
Abstract classes are a special type of base classes. In addition to normal class members, they have abstract class members. These Abstract class members are methods and properties that are declared without an implementation. All classes derived directly from abstract classes must implement all of these abstract methods and properties.
Abstract classes can never be instantiated. This would be illogical, because of the members without implementations.So what good is a class that can't be instantiated? Lots! Abstract classes sit toward the top of a class hierarchy. They establish structure and meaning to code. They make frameworks easier to build. This is possible because abstract classes have information and behavior common to all derived classes in a framework. Take a look at the following example:
abstract public class Contact // Abstract Class Contact.{protected string name;public Contact()
{// statements...}public abstract void generateReport();abstract public string Name
{get;set;
}
}
Contact, is an abstract class. Contact has two abstract members, and it has an abstract method named generateReport(). This method is declared with the abstract modifier in front of the method declaration. It has no implementation (no braces) and is terminated with a semicolon. The Name property is also declared abstract. The accessors of properties are terminated with semicolons.
public class Customer : Contact // Customer Inherits Abstract Class Contact.{string gender;decimal income;int numberOfVisits;public Customer()
{// statements}public override void generateReport()
{// unique report}public override string Name
{get{
numberOfVisits++;return name;
}set{
name = value;
numberOfVisits = 0;
}
}
}public class SiteOwner : Contact
{int siteHits;string mySite;public SiteOwner()
{// statements}public override void generateReport()
{// unique report}public override string Name
{get{
siteHits++;return name;
}set{
name = value;
siteHits = 0;
}
}
}
The abstract base class Contact has two derived classes, Customer and SiteOwner. Both of these derived classes implement the abstract members of the Contact class. The generateReport() method in each derived class has an override modifier in its declaration. Likewise, the Name declaration contains an override modifier in both Customer and SiteOwner.
C# requires explicit declaration of intent when overriding methods. This feature promotes safe code by avoiding the accidental overriding of base class methods, which is what actually does happen in other languages. Leaving out the override modifier generates an error. Similarly, adding a new modifier also generates an error. Abstract methods must be overridden and cannot be hidden, which the new modifier or the lack of a modifier would be trying to do.
The most famous of all abstract classes is the Object class. It may be referred to as object or Object, but it's still the same class. Object is the base class for all other classes in C#. It's also the default base class when a base class is not specified. The following class declarations produce the same exact results:
abstract public class Contact : Object
{// class members}abstract public class Contact
{// class members}
Object is implicitly included as a base class if it is not already declared. Besides providing the abstract glue to hold together the C# class framework, object includes built-in functionality, some of which is useful for derived classes to implement.
By Milan Saxena
Category:
C#,
OOPS
0
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