Flyweight Pattern is one of the Structural Design Patterns. Its goal is to ease the memory consumption when using many objects by extracting the common properties in some kind of pool in the memory, represented by a data structure such as Dictionary, HashMap or another.
Real World Example
Lets say, for example that you are creating a Car Registration System. Everything is fine until you have to dealt with the problem of registering millions of cars and your system begins to take large portion of memory.
One way to deal with this problem is to use Flyweight Pattern.
Every car has properties like model, year, make, owner, tag, renew date and etc.. Some of these properties are common among the cars and you could extract them so the memory for them is allocated only once and shared between the cars. This is one classical example of Flyweight Pattern usage and it should decrease your system memory consumption.
Example Implementation
So, what we are going to need to make our Car System work efficiently.
We will need a CarFactory, CarManager and two objects to hold the intrinsic (common) and extrinsic (variable) car properties.
Normally we would use Interfaces for the factory and the manager, but for sake of simplicity we will not, in this post.
I use the two objects to avoid confusion to which properties are intrinsic and which are extrinsic.
Now lets take a look at CarFactory. It holds a Dictionary that holds the common intrinsic car properties, to avoid the instantiation of multiple common objects.
And finally here is our CarManager, where the actual registration logic is.