Question

Say I store a object in a hashmap that has a atomiclong as property.

Now I want to put a key in the hashmap if it doesn’t exist, and update the value if it does.

When I get the object using the key, I will increment the property which is an atomic long.

What I need to understand is, what operation is thread-safe here?

Psuedo code:

HashMap hm = new HashMap

if(hm.containsKey(key1)) {
  MyCounter counter = (MyCounter)hm.get(key1);
  counter.incrementAndGet();  
}
else {
  MyCounter newCounter = new MyCounter();
  newCounter.incrementAndGet();
  hm.put(key1, newCounter);
}

Answer 1

I would use a synchronized block as that is much simpler.

Map<KeyType, AtomicInteger> map = ...

synchronized(map) {
   AtomicInteger count = map.get(key);
   if (count == null)
       map.put(key, count = new AtomicInteger());
   count.incrementAndGet();
}

The cost of synchronized is unlikely to be enough to make it worth complicating your solution.