Flyweight Pattern

Hi!

Please explain about Flyweight Pattern

From this site:http://www.allapplabs.com/java_desig...ht_pattern.htm


The typical example you can see on this in every book will be of folders. The folder with name of each of the company employee on it, so, the attributes of class Folder are: ‘Selected’ , ‘Not Selected’ and the third one is ‘employeeName’. With this methodology, we will have to create 2000 folder class instances for each of the employees.

I cann't understand this example..May you someone expalin more about that

The article is poorly written, but did you read the rest of it about the grinding wheel example?

Have you tried Googling for other examples?

Exactly what is it that you don't understand?

One can think effectively only when one is willing to endure suspense and to undergo the trouble of searching...
J. Dewey

The concept of the FlyWeight pattern is very simple. It is a concrete (instanciatable) class that has (ideally) NO data members. It exists to encapsulate FUNCTIONALLITY but NOT state.

Because they take little or no memory, it becomes posible to instanciate a great quantity of these instances.

I cann't understand the code in that article,I need explain by code!

Quote:

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Originally Posted by Abalfazl

I cann't understand the code in that article,I need explain by code!

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Probably because that is such a poor example.

Consider instead a Mailing list. One design would be to implement a class which maintained the state for an individual addresse, along with a collection of these objects.

If you werre to use a flyweight patterns. You would maintain the state in the collection itself, and the individual addressee class would implement its accessors by referencing the collection.

psuedo-code

Code:

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class addressee
{
  private int ID;
  public string get_Name { return MailingList.get_Name(ID); }
}

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The size of each addressee now drops from a fairly large object (say 4-7 fields of up to 50 characters) down to just the side of an integer.

This makes it easy to replace an im-memory store of the mailing list collection, with a file or database store, and eliminate the memory utilization without having to go and re-work your addressee class.

http://www.developer.com/design/article.php/3677501

If I use the flyweight pattern so that I only store unique Time instances, I can reduce the maximum number of typical time objects to 96 (24 hours times four possible times per hour). The appointment application will still support odd times, but predictable use suggests that flyweight will dramatically minimize object creation and memory usage.

When we use flyweight we share common data between objects,Which data in these objects iin this example are similar that he share?

Quote:

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Originally Posted by Abalfazl

Which data in these objects iin this example are similar that he share?

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In that example whole Time objects are shared.

Time objects are not created using new directly. Instead a factory method is called. The factory method keeps track of all Time objects it creates. If a specific Time object already has been created before, the factory method returns a reference to this existing object rather than creating a new one. So if you do,

Time t1 = TimeFactory.create(12, 50);
Time t2 = TimeFactory.create(12, 50);

the factory method (called create) will make sure both t1 and t2 refers to the same object. So t1==t2 will be true. This is the Flyweight sharing.

Note that String literals are treated according to the same principle. If you do,

String s1 = "hello";
String s2 = "hello";

Then both s1 and s2 will refer to the same String object. So s1==s2 will be true.

Also note that this kind of Flyweight sharing requires objects to be immutable (after creation the object state cannot be changed).

Integers are also treated the same way.

I found it interesting to open up the code for Integer. You'll see that the first call goes through the first hundred or so integers and creates objects for them. Then each subsequent call returns that object.

This way if you have 1000 Integer(0) objects in reality its just one. Its an excellent way to save memory.

Quote:

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Originally Posted by compavalanche

I found it interesting to open up the code for Integer.

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In fact this is not true just for the particular Java implementation you happened to look at. This behaviour is defined by the Java language. The JLS states that Integers in the -128 to 127 range are to be given the Flyweight sharing treatment (but it doesn't use these exact words :)).