The Flyweight Pattern is a pattern for greatly reducing memory requirements by not requiring that heavy-weight objects be created in large numbers when dealing with systems that contain many things that are mostly the same. If for instance, a document was modeled using a complex character class that knew about unicode, fonts, positioning, etc., then the memory requirements could be quite large for large documents if each physical character in the document required its own character class instance. Instead, characters themselves might be kept within Strings and we might have one character class (or a small number such as one character class for each font type) that knew the specifics of how to deal with characters.
In such circumstances, we call the state that is shared with many other things (e.g. the character type) instrinsic state. It is captured within the heavy-weight class. The state which distinguishes the physical character (maybe just its ASCII code or Unicode) is called its extrinsic state.
First we are going to model some complex aircraft (the first being a hoax competitor of the second - not that is relevant to the example).
If we want to model our fleet, our first attempt might involve using many instances of these heavy-weight objects. It turns out though that only a few small pieces of state (our extrinsic state) change for each aircraft, so we will have singletons for the heavy-weight objects and capture the extrinsic state (bought date and asset number in the code below) separately.
So here, even if our fleet contained hundreds of planes, we would only have one heavy-weight object for each type of aircraft.
As a further efficiency measure, we might use lazy creation of the flyweight objects rather than create the initial map up front as in the above example.
Running this script results in: