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Decorator Pattern
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INTRODUCTION

The decorator pattern works by Wrapping the new "decorator" object around the original object, which is typically achieved by passing the original object as a parameter to the constructor of the decorator, with the decorator implementing the new functionality. The interface of the original object needs to be maintained by the decorator.

Decorators are alternatives to Subclass ing. Subclassing adds behaviour at compile time whereas decorators provide a new behaviour at runtime.

This difference becomes most important when there are several ''independent'' ways of extending functionality. Since (in most Object-oriented Programming Language s) Classes cannot be created at Runtime and it is typically not possible to predict what combinations of extensions will be needed at design time, a new class would have to be made for every possible combination. By contrast, decorators are objects, created at runtime, and can be combined on a per-use basis. A superb example of the decorator pattern is the Java I/O Streams implementation.


MOTIVATION

As an example, consider a window in a Windowing System . To allow Scrolling of the window's contents, we may wish to add horizontal or vertical Scrollbar s to it, as appropriate. Assume windows are represented by instances of the ''Window'' class, and assume this class has no functionality for adding scrollbars. We could create a subclass ''ScrollingWindow'' that provides them, or we could create a ''ScrollingWindowDecorator'' that merely adds this functionality to existing ''Window'' objects. At this point, either solution would be fine. (Another solution is to simply modify the existing ''Window'' class, but this is not always possible—we might not have access to its implementation, or we might be adding Storage overhead for new functionality that the majority of objects will not use.)

Now let's assume we also wish the option to add borders to our windows. Again, our original ''Window'' class has no support. The ''ScrollingWindow'' subclass now poses a problem, because it has effectively created a new kind of window. If we wish to add border support to ''all'' windows, we must create subclasses ''WindowWithBorder'' and ''ScrollingWindowWithBorder''. Obviously, this problem gets worse with every new feature to be added. For the decorator solution, we need merely create a new ''BorderedWindowDecorator''—at runtime, we can decorate existing windows with the ''ScrollingWindowDecorator'' or the ''BorderedWindowDecorator'' or both, as we see fit.


APPLICABILITY

Consider the webpage you are currently viewing. The webpage itself displays the information you need, but your web browser knows nothing about the content. It is likely that this webpage doesn't fit in the entire browser area and a scrollbar is required to show the information. The web browser doesn't need to assume that all webpages will require a scrollbar and it certainly should never assume a scrollbar is never needed. Mozilla and Netscape browsers will display the scrollbar only if it is necessary and hides it if it is unnecessary. In this case, the scrollbar is the "decoration" to the webpage. It takes care of whether it should be displayed dynamically as opposed to statically forcing the webpage display to be a subclass of the scrollbar display. Thus, it is up to the scrollbar to decide whether it should display itself (instead of trying to force that responsibility on the webpage or on the external parts of the web browser).


EXAMPLE

This Java example uses the window/scrolling scenario.


// the Window interface

interface Window {

    public void draw(); // draws the Window

    public String getDescription(); // returns a description of the Window

}



// implementation of a simple Window without any scrollbars

class SimpleWindow implements Window {

    public void draw() {

        // draw window

    }



    public String getDescription() {

        return "simple window";

    }

}


The following classes contain the decorators for all Window classes, including the decorator classes themselves.



// abstract decorator class - note that it implements Window

abstract class WindowDecorator implements Window {

    protected Window decoratedWindow; // the Window being decorated



    public WindowDecorator(Window decoratedWindow) {

        this.decoratedWindow = decoratedWindow;

    }

}



// the first concrete decorator which adds vertical scrollbar functionaliy

class VerticalScrollBarDecorator extends WindowDecorator {

    public VerticalScrollBarDecorator (Window decoratedWindow) {

        super(decoratedWindow);

    }

 

    public void draw() {

        drawVerticalScrollBar();

        decoratedWindow.draw();

    }



    private void drawVerticalScrollBar() {

        // draw the vertical scrollbar

    }



    public String getDescription() {

        return decoratedWindow.getDescription() + ", including vertical scrollbars";

    }

}



// the second concrete decorator which adds horizontal scrollbar functionaliy

class HorizontalScrollBarDecorator extends WindowDecorator {

    public HorizontalScrollBarDecorator (Window decoratedWindow) {

        super(decoratedWindow);

    }



    public void draw() {

        drawHorizontalScrollBar();

        decoratedWindow.draw();

    }



    private void drawHorizontalScrollBar() {

        // draw the horizontal scrollbar

    }



    public String getDescription() {

        return decoratedWindow.getDescription() + ", including horizontal scrollbars";

    }

}


Here's a test program that creates a Window instance which is fully decorated (i.e., with vertical and horizontal scrollbars), and prints its description:



public class DecoratedWindowTest {

    public static void main(String {Link without Title}  args) {

        // create a decorated Window with horizonal and vertical scrollbars

        Window decoratedWindow = new HorizontalScrollBarDecorator(

                new VerticalScrollBarDecorator(new SimpleWindow()));



        // print the Window's description

        System.out.println(decoratedWindow.getDescription());

    }

}


The output of this program is "simple window, including vertical scrollbars, including horizontal scrollbars". Notice how the getDescription method of the two decorators first retrieve the decorated Window's description and "decorates" it with a suffix.


SEE ALSO



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