Decorator pattern

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In object-oriented programming, the decorator pattern is a design pattern that allows new/additional behavior to be added to an existing method of an object dynamically.

UML Class diagram of the decorator pattern

Contents

1 Introduction
2 Motivation
3 Applicability
4 Example
5 See also
6 External links

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 subclassing. 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 languages) 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 scrollbars 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 


// implementation of a simple Window without any scrollbars
class SimpleWindow implements Window

public String getDescription()

}

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

}

// the first concrete decorator which adds vertical scrollbar functionality class VerticalScrollBarDecorator extends WindowDecorator

public void draw()

private void drawVerticalScrollBar()

public String getDescription()

}

// the second concrete decorator which adds horizontal scrollbar functionality class HorizontalScrollBarDecorator extends WindowDecorator

public void draw()

private void drawHorizontalScrollBar()

public String getDescription()

}

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

}

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.

External links

[Description] by Vince Huston
Sample "[Chapter 3: The Decorator Pattern]"
Sample Chapter "[C# Design Patterns: The Decorator Pattern]" by James W. Cooper
[A PHP approach] and "[Redecorated]"
[A Delphi approach]
Article "[Three approaches for decorating your code]" by Michael Feldman
Article "[Using the Decorator Pattern]" by Budi Kurniawan
Article "[The Decorator Design Pattern]" by Antonio García and Stephen Wong
[Application] with Gilgul

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