Flyweight Design Pattern Principle

Flyweight pattern is a structural pattern in GoF as it supports large numbers of fine-grained objects efficiently. Since each fine-grained object (e.g. a character in an editor application) may incur overhead that adds up. This pattern shares objects in a pool (Flyweight Factory) to minimize memory use.

source: GoF

overview

Flyweight: A flyweight is a shared independent object with its own intrinsic state (e.g. character code) that is stateless or immutable. If the intrinsic state is not going to be modified, then the object can be shared. Flyweight objects are shared among different contexts that hold extrinsic states (e.g. coordinate position in the document and its typographic style). These extrinsic state (out side of object) of the context will be passed by client object to the flyweight to operate. A flyweight makes as much of its state information extrinsic as possible and that is where the memory saving comes from.

FlyweightFactory: Once you have flyweight, then you can have a FlyweightFactory as a Flyweight Pool which is a collection of Flyweights that are cached to be shared.  FlyweightFactory is usually a Singleton to keep track of whether a particular flyweight has been generated and returns either a new instance or reference to one that it has already generated.

Real world Flyweight use cases: 

1. Apply Flylweight to cells on a big board. For example, each cell on a 1000 by 1000 game of life game board can be a flyweight object to reduce memory footprint for each cell since extrinsic state are kelp outside of the Flyweight object.
2. The Java language specification requires that integers between -128 and 127 be 'cached' is actually a form of flyweight to conserve system resource.

flyweight code example

FlyweightFactory class:

Flyweight class:

source: ​Java™ Design Patterns: A Tutorial

when to use

1. Need use large numbers of objects when a simple repeated shared representation would save a large amount of memory.​​​
2. Not often used at the application level in Java, more of a system resource management technique that is used at a lower level than Java.
   

comparison

Flyweight vs. Composite

• Flyweight tends to go with composite that contains Flyweight objects.

Flyweight vs. Strategy

• Flyweights are small strategy classes handled by the flyweight factory.
• Both Strategy and Flyweight externalize extrinsic state.

references

• The Game of Life