Question

I was going to implement some code which needs a synchronized data structure. I came up with HashTable and Collections.synchronized(HashMap). I wouldn’t be needing ConcurrentHashMap for this. I was wondering which one of the two would be better.

PS : I will be calling a lot of getter of this object and they would not be at the same time. So their is no problem with concurrency issue also.

Answer 1

ConcurrentHashMap is much more scaleable: http://www.javamex.com/tutorials/concurrenthashmap_scalability.shtml

HashTable and Collections.synchronized(HashMap) provide with the same performance, but they are conditionally thread-safe (i.e. they are not fully thread-safe)

If there are a lot read operations, I would recommend to wrap it with read-write locks:

public class MyHashMap<K, V> extends HashMap<K, V> {
     private final ReadWriteLock lock = new ReentrantReadWriteLock();

    @Override
    public V put(K key, V value) {
        final Lock w = lock.writeLock();
        w.lock();
        try {
            return super.put(key, value);
        } finally {
            w.unlock();
        }
    }

    @Override
    public V get(Object key) {
        final Lock r = lock.readLock();
        r.lock();
        try {
            return super.get(key);
        } finally {
            r.unlock();
        }
    }
    .... // the same approach distinguishing read and write operations

}

UPDATE:

I will be calling a lot of getter of this object and they would not be at the same time

It doesn’t guarantee that you don’t need synchronization.