public Class Flat3Map<K, V>

This map is designed for performance and can outstrip HashMap. It also has good garbage collection characteristics.

• Optimised for operation at size 3 or less.
• Still works well once size 3 exceeded.
• Gets at size 3 or less are about 0-10% faster than HashMap,
• Puts at size 3 or less are over 4 times faster than HashMap.
• Performance 5% slower than HashMap once size 3 exceeded once.

The design uses two distinct modes of operation - flat and delegate. While the map is size 3 or less, operations map straight onto fields using switch statements. Once size 4 is reached, the map switches to delegate mode and only switches back when cleared. In delegate mode, all operations are forwarded straight to a HashMap resulting in the 5% performance loss.

The performance gains on puts are due to not needing to create a Map Entry object. This is a large saving not only in performance but in garbage collection.

Whilst in flat mode this map is also easy for the garbage collector to dispatch. This is because it contains no complex objects or arrays which slow the progress.

Do not use Flat3Map if the size is likely to grow beyond 3.

Note that Flat3Map is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. The simplest approach is to wrap this map using java.util.Collections#synchronizedMap(Map). This class may throw exceptions when accessed by concurrent threads without synchronization.