# Flyweight Pattern

version 2.5.18

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 modelled 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.

## 1. Example

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).

    class Boeing797 {
        def wingspan = '80.8 m'
        def capacity = 1000
        def speed = '1046 km/h'
        def range = '14400 km'
        // ...
    }

    class Airbus380 {
        def wingspan = '79.8 m'
        def capacity = 555
        def speed = '912 km/h'
        def range = '10370 km'
        // ...
    }

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.

    class FlyweightFactory {
        static instances = [797: new Boeing797(), 380: new Airbus380()]
    }

    class Aircraft {
        private type         // instrinsic state
        private assetNumber  // extrinsic state
        private bought       // extrinsic state
        Aircraft(typeCode, assetNumber, bought) {
            type = FlyweightFactory.instances[typeCode]
            this.assetNumber = assetNumber
            this.bought = bought
        }
        def describe() {
            println """
            Asset Number: $assetNumber
            Capacity: $type.capacity people
            Speed: $type.speed
            Range: $type.range
            Bought: $bought
            """
        }
    }

    def fleet = [
        new Aircraft(380, 1001, '10-May-2007'),
        new Aircraft(380, 1002, '10-Nov-2007'),
        new Aircraft(797, 1003, '10-May-2008'),
        new Aircraft(797, 1004, '10-Nov-2008')
    ]

    fleet.each { p -> p.describe() }

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:

    Asset Number: 1001
    Capacity: 555 people
    Speed: 912 km/h
    Range: 10370 km
    Bought: 10-May-2007

    Asset Number: 1002
    Capacity: 555 people
    Speed: 912 km/h
    Range: 10370 km
    Bought: 10-Nov-2007

    Asset Number: 1003
    Capacity: 1000 people
    Speed: 1046 km/h
    Range: 14400 km
    Bought: 10-May-2008

    Asset Number: 1004
    Capacity: 1000 people
    Speed: 1046 km/h
    Range: 14400 km
    Bought: 10-Nov-2008