Thursday, 21 August 2014

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[].
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