Design Patterns. Singleton. Real life examples?

Hello All,
1. 'There is no substitute for experience.' So would it be
possible that you share your experiences by providing an example
of real life application of the Singleton pattern.
2. 1M thanks for all.

I'll give you two Real Life examples in a computer environment:
You can use the singleton pattern wherever you have to share one device (e.g. a DB connection, a serial port or a file access) among multiple instances of a class. So the singleton has the job to control the access to a limited resource.
You can use the singleton pattern also as "Yellow Pages". Assuming you plan a big project with a lot of modules and you don't know at developing time how much and what kind of modules will be on your destination system. So you can use the singleton pattern in a, let's say "Library"-class. Assume that each module will be registered by that library, thus it is possible to perform requests for modules (maybe also by other modules) to that library. The library should exist on a system only once.
I hope these examples helps.
J�rg

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Roronoa_Zorro wrote:
Hello , this is my first time posting and I hope to be doing it in the right section. I have a school assignment that says the following: Identify real or abstract structures that could be modeled by the class diagrams represented below: [http://img704.imageshack.us/img704/3523/diagrams.png]
Upon searching I found out that this is a composite design pattern. I have to implement 2 scenarios for this class system. I was thinking of taking class A (from the uploaded image) to be the Software ( more like a program oriented towards a certain domain like multimedia ), B1,B2 could be an Application/Compiler, and the composite class (D) is an Operating System( because is is usually compiled out of many software (program) pieces) . I want to ask for your opinions on this example because I'm not sure whether this is correct or not. If it isn't could you please give me a real-life example of a correct model ?.Unless your assignment specifically required you to use a computer analogy I would use something else, and even then, I'd still use something else first to understand it better.Since OperatingSystem objects aren't composed with other OperatingSystem objects this doesn't really fit the pattern. Now of course you may be able to run several virtual operating systems inside one but that stretches the example too much for my taste. A directory tree may be seen as an example of the composite pattern -- class A can be a Node, different types of files are specializations of a Node (B1, B2, ...) and are not composable (can't put one file inside another), and a Directory (D) is a Composable specialization of a Node -- it can contain other Nodes. [Edit: I see Saish beat me to the file system analogy]
With real-world examples you have to be careful as some obvious examples that seem to correspond to computer world, such as containers, boxes, etc, don't actually work, e.g. boxes aren't composable because if box1 fits into box2, then box2 won't fit into box1. Power extension cords and surge protectors are composable extensions of Pluggable class, though not recommended that you take advantage of that feature too much lest you burn your house down. Bags are composable in the real world since they aren't rigid and can be folded up -- class A would just be Object, class B1, B2, etc would be stuff that isn't composable like Apple and Orange, and D would be Bag. You can put Apple, Orange, and Bag instances inside another Bag instance.
Maybe we should have a contest here -- who can come up with the most creative real-world analogy to the Composite pattern? I know mine are pretty lame.
Edited by: fromrussiawithjava on Mar 30, 2010 5:36 AM

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Moderator: Please, respect the rules of the forum

What you are asking is very project specific.
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Design Patterns. Singleton. Real life examples?

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