Java : MultiKeyMap Example

•  A Map implementation that uses multiple keys to map the value.
• This class is the most efficient way to uses multiple keys to map to a value. The best way to use this class is via the additional map-style methods. These provide get, containsKey, put and remove for individual keys which operate without extra object creation.
• The additional methods are the main interface of this map. As such, you will not normally hold this map in a variable of type Map.
• The normal map methods take in and return a MultiKey. If you try to use put() with any other object type a ClassCastException is thrown. If you try to use null as the key in put() a NullPointerException is thrown.
• This map is implemented as a decorator of a AbstractHashedMap which enables extra behaviour to be added easily.
•  MultiKeyMap.decorate(new LinkedMap()) creates an ordered map
•  MultiKeyMap.decorate(new LRUMap()) creates an least recently used map.
• MultiKeyMap.decorate(new ReferenceMap()) creates a garbage collector sensitive map. 
• Note that IdentityMap and ReferenceIdentityMap are unsuitable for use as the key comparison would work on the whole MultiKey, not the elements within.
• Note that MultiKeyMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. This class may throw exceptions when accessed by concurrent threads without synchronization.
• Apache Commons Collections to create a MultiKeyMap that will store two keys with one corresponding value and then using MapIterator to walk through the map.

• MultiKeyMap keys are instances of MultiKey, which has a getKey(int index) method. There is also a getKeys() method which returns Object[].