When do we need decorator pattern?
Two real-life examples:
The item upgrading systems in Diablo 2 and Final Fantasy 7. Weapons and armor have sockets or slots. During the game, the player put upgrades (gems, runes or materia) into those slots. Each upgrade has an individual effect (say, 8 points fire damage or 10% mana leech). So when you swing your sword, it does its base damage plus the damage added by each upgrade you've added. This matches the decorator pattern extremely closely.
From Gang of Four:
A graphical user interface toolkit, for example, should let you add properties like borders or behaviors like scrolling to any user interface component.
...
The decorator conforms to the interface of the component it decorates so that its presence is transparent to the component's clients. The decorator forwards requests to the component and may perform additional actions (such as drawing a border) before or after forwarding. Transparency lets you nest decorators recursively, thereby allowing an unlimited number of added responsibilities.
The Streams in Java - subclasses of InputStream and OutputStream are perfect examples of the decorator pattern.
As an example, writing a file to disk:
File toWriteTo = new File("C:\\temp\\tempFile.txt");
OutputStream outputStream = new FileOutputStream(toWriteTo);
outputStream.write("Sample text".getBytes());
Then should you require some extra functionality regarding the writing to disk:
File toWriteTo = new File("C:\\temp\\tempFile.txt");
OutputStream outputStream =
new GZIPOutputStream(new FileOutputStream(toWriteTo));
outputStream.write("Sample text".getBytes());
By simply "chaining" the constructors, you can create quite powerful ways of writing to disk. The beauty in this way is that you can add different (in this example) OutputStream implementations later on. Also, each implementation doesn't know how the others work - they all just work to the same contract. This also makes testing each implementation very easy in isolation.
There are plenty of "real world" examples of where the decorator pattern can be used. Off the top of my head, some examples:
- Reading and writing to disk (above)
- Construction of UI elements, such as adding scrollbars on to text areas etc
Head First Design Patterns has some more "real world" examples. It seems that O'Reilly has their sample chapter, which is on Decorator Pattern, for free; Google showed up this link: PDF