Map<KeyObject, Object> cacheMap = new HashMap<KeyObject, Object>();
Map<KeyObject, Integer> visiteCountMap = new HashMap<KeyObject, Integer>();
int cacheSize = 100;
/**
* retrivee object.
*/
Object retrieve(KeyObject key){
     Object obj = null;
     if(cacheMap.containsKey(key)){
     obj = cacheMap.get(key);
     } else {
     //find given key's object from other way.

     obj = service.findObject(key);
     if(null != obj){
         cacheMap.put(key, obj);
        }
     }
         
     //increate count.

     if(visiteCountMap.containsKey(key)){
     visiteCountMap.put(key, visiteCountMap.get(key) + 1);
     } else {
     visiteCountMap.put(key, new Integer(1));
     }
    
     //eliminate some cached object while size >= 100.

     if(cacheMap.size() >= cacheSize){
     eliminate();
     }
    
     return obj;
}
/**
* eliminate object that not visited usually.
*/
void eliminate(){
    Set<KeyObject> keys = cacheMap.keySet();
    if(null != keys){
     for(KeyObject key : keys){
         if(visiteCountMap.get(key).intValue < 5){
             //eliminate the object.

                cacheMap.remove(key);
            }
        }
    }
}