Question

I have a collection of objects that is modified by one thread and read by another (more specifically the EDT). I needed a solution that gave me fast look up and also fast indexing (by order inserted), so I’m using a ConcurrentHashMap with an accompanying ArrayList of the keys, so if want to index an entry, I can index the List for the key and then use the returned key to get the value from the hash map. So I have a wrapper class that makes sure when and entry is added, the mapping is added to the hash map and the key is added to the list at the same time, similarly for removal.

I’m posting an example of the code in question:

private List<K> keys = Collections.synchronizedList(new ArrayList<K>(INITIAL_CAPACITY));

private ConcurrentMap<K, T> entries = new ConcurrentHashMap<K, T>(INITIAL_CAPACITY, .75f);

public synchronized T getEntryAt(int index){
     return entries.get(keys.get(index));
}

**public synchronized void addOrReplaceEntry(K key, T value){
     T result = entries.get(key);
     if(result == null){
         entries.putIfAbsent(key, value);
         keys.add(key);
     }
     else{
         entries.replace(key, result);
     }
}**

public syncrhonized T removeEntry(K key, T value){
     keys.remove(key);
     entries.remove(key, value);
}

public synchronized int getSize(){
     return keys.size();
}

my question is: am I losing all the benefits of using the ConcurrentHashMap (over syncrhonized hashmap) by operating on it in synchronized methods? I have to synchronize the methods to safely modify/read from the ArrayList of keys (CopyOnWriteArrayList is not an option because a lot of modification happens…) Also, if you know of a better way to do this, that would be appreciated…

Answer 1

Yes, using a Concurrent collection and a Synchronized collection in only synchronized blocks is a waste. You wont get the benefits of ConcurrentHashMap because only one thread will be accesing it at a time.

You could have a look at this implementation of a concurrent linked hashmap, I havnt use it so can’t attest to it’s features.

One thing to consider would be to switching from synchronized blocks to a ReadWriteLock to improve concurrent read only performance.

I’m not really sure of the utility of proving a remove at index method, perhaps you could give some more details about the problem you are trying to solve?