11.4 Abstract Classes

Each type of Window has a different shape and appearance. Drop-down listboxes look very different from buttons. Clearly, every subclass of Window should implement its own DrawWindow() method — but so far, nothing in the Window 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.

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 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.

The Idea Behind Abstraction 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 Window class ought to lay out the common characteristics and behaviors of all windows, even though you never intend to instantiate the abstraction Window itself. The idea of an abstract class is implied in the word "abstract." It serves to implement the abstraction "Window" that will be manifest in the various concrete instances of Window, such as browser window, frame, button, listbox, drop-down, and so forth. The abstract class establishes what a Window is, even though we never intend to create a "Window" per se. An alternative to using abstract is to define an interface, as described in Chapter 14.

Thus, if you were to designate DrawWindow() as an abstract method in the Window class, the Window class would become abstract. Then you could derive from Window, but you could not create any Window instances. If the Window class is an abstraction, there is no such thing as a simple Window object, only objects derived from Window.

Making Window.DrawWindow() abstract means that each class derived from Window would have to implement its own DrawWindow() 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.

Designating a method as abstract is accomplished by placing the abstract keyword at the beginning of the method definition, as follows:

abstract public void DrawWindow();

(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:

abstract public class Window

Example 11-3 illustrates the creation of an abstract Window class and an abstract DrawWindow() method.

Example 11-3. Abstract methods

using System;

abstract public class Window
{
    // constructor takes two integers to
    // fix location on the console
    public Window(int top, int left)
    {
        this.top = top;
        this.left = left;
    }

    // simulates drawing the window
    // notice: no implementation
    abstract public void DrawWindow();

    protected int top;
    protected int left;
    
}

// ListBox derives from Window
public class ListBox : Window
{
    // constructor adds a parameter
    public ListBox(
        int top, 
        int left, 
        string contents):
        base(top, left)  // call base constructor
    {
 
        listBoxContents = contents;
    }
    
    // an overridden version implementing the
    // abstract method
    public override void DrawWindow()
    {
       
        Console.WriteLine ("Writing string to the listbox: {0}", 
            listBoxContents);
    }

    private string listBoxContents;  // new member variable
}

public class Button : Window
{
    public Button(
        int top,
        int left):
        base(top, left)
    {
    }

    // implement the abstract method
    public override void DrawWindow()
    {
        Console.WriteLine("Drawing a button at {0}, {1}\n",
            top, left);
    }

}

public class Tester
{
    static void Main()
    {
        Window[] winArray = new Window[3];
        winArray[0] = new ListBox(1,2,"First List Box");
        winArray[1] = new ListBox(3,4,"Second List Box");
        winArray[2] = new Button(5,6);

        for (int i = 0;i < 3; i++)
        {
            winArray[i].DrawWindow();
        }
    }
}
Writing string to the listbox: First List Box
Writing string to the listbox: Second List Box
Drawing a button at 5, 6

In Example 11-3, the Window class has been declared abstract and therefore cannot be instantiated. If you replace the first array member:

winArray[0] = new ListBox(1,2,"First List Box");

with this code:

winArray[0] = new Window(1,2);

the program generates the following error:

Cannot create an instance of the abstract class or interface 'Window'

You can instantiate the ListBox and Button objects because these classes override the abstract method, thus making the classes concrete (i.e., not abstract).