Java Concurrency Incrementing a Value

Question

I have been reading about volatile and synchronized in Java but have been scratching my head in confusion. I am hoping someone can help me clear up

Answer 1

The answer is of course to use AtomicInteger, not int.

And if you use a ConcurrentHashMap, you need to use its special thread-safe method putIfAbsent().

Here's a javadoc excerpt from that method:

This is equivalent to

if (!map.containsKey(key))
    return map.put(key, value);
else
    return map.get(key);

except that the action is performed atomically.

private ConcurrentMap map = new ConcurrentHashMap();

AtomicInteger value = map.get("value");
if (value == null) {
    map.putIfAbsent("value", new AtomicInteger());
    value = map.get("value");
}
value.incrementAndGet();

Note how you don't have to put the new value back.

If you make that change to your code, it should all work just fine.