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Abstract Classes
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Each type of Control has a different shape and appearance. Drop-down ListBoxes look very different from Buttons. Clearly, every subclass of Control should implement its own DrawControl( ) method but so far, nothing in the Control class enforces that they must do so. To require subclasses to implement a method of their base, you need to designate that method as abstract, rather than virtual. An abstract method has no implementation. It creates a method name and signature that must be implemented in all derived classes. Furthermore, making at least one method of any class abstract has the side effect of making the entire class abstract. Abstract classes establish a base for derived classes, but it is not legal to instantiate an object of an abstract class. Once you declare a method to be abstract, you prohibit the creation of any instances of that class. Thus, if you were to designate DrawControl( ) as an abstract method in the Control class, the Control class itself would become abstract. Then you could derive from Control, but you could not create any Control instances. That makes sense, because the Control class is an abstraction there is no such thing as a simple Control object, only objects derived from Control. Making Control.DrawControl( ) abstract means that each class derived from Control would have to implement its own DrawControl( ) method. If the derived class failed to implement the abstract method, that derived class would also be abstract, and again no instances would be possible.
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The Idea Behind Abstraction
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Abstract classes should not just be an implementation trick; they should represent the idea of an abstraction that establishes a contract for all derived classes. In other words, abstract classes mandate the public methods of the classes that will implement the abstraction. The idea of an abstract Control class ought to lay out the common characteristics and behaviors of all Controls, even though you never intend to instantiate the abstraction Control itself. The idea of an abstract class is implied in the word abstract. It serves to implement the abstraction Control that will be manifest in the various concrete instances of Control, such as button, listbox, drop-down, and so forth. The abstract class establishes what a Control is, even though you never intend to create a plain Control by itself. An alternative to using abstract is to define an interface, as we describe in 13.
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11: Inheritance and Polymorphism
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You designate a method as abstract simply by placing the abstract keyword at the beginning of the method definition:
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abstract public void DrawControl( );
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(Because the method can have no implementation, there are no braces, only a semicolon.) If one or more methods are abstract, the class definition must also be marked abstract, as in the following:
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public abstract class Control
Example 11-3 illustrates the creation of an abstract Control class and an abstract DrawControl( ) method.
using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_11_3_ _ _ _Abstract_Methods { public abstract class Control { protected int top; protected int left; // constructor takes two integers to // fix location on the console public Control(int top, int left) { this.top = top; this.left = left; } // simulates drawing the control // notice: no implementation public abstract void DrawControl( ); } // end class Control
// ListBox derives from Control public class ListBox : Control { private string listBoxContents; // new member variable // constructor adds a parameter public ListBox( int top, int left, string contents) : base(top, left) // call base constructor {
Abstract Classes |
listBoxContents = contents; } // an overridden version implementing the // abstract method public override void DrawControl( ) { Console.WriteLine("Writing string to the listbox: {0}", listBoxContents); } // end class ListBox
}
public class Button : Control { public Button( int top, int left) : base(top, left) { } // override the abstract method public override void DrawControl( ) { Console.WriteLine("Drawing a button at {0}, {1}\n", top, left); } // end class Button
}
public class Tester { static void Main( ) { Control[] controlArray = new Control[3]; controlArray[0] = new ListBox(1, 2, "First ListBox"); controlArray[1] = new ListBox(3, 4, "Second ListBox"); controlArray[2] = new Button(5, 6); for (int i = 0; i < 3; i++) { controlArray[i].DrawControl( ); } // end for loop // end main // end class Tester
} } }
The output looks like this:
Writing string to the listbox: First ListBox Writing string to the listbox: Second ListBox Drawing a button at 5, 6
In Example 11-3, the Control class has been declared abstract and therefore cannot be instantiated. If you replace the first array member:
controlArray[0] = new ListBox(1,2,"First ListBox");
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