java concurrency: many writers, one reader

Question

I need to gather some statistics in my software and i am trying to make it fast and correct, which is not easy (for me!)

first my code so far with two classes, a StatsS

Answer 1

As jack was eluding to you can use the java.util.concurrent library which includes a ConcurrentHashMap and AtomicLong. You can put the AtomicLong in if absent else, you can increment the value. Since AtomicLong is thread safe you will be able to increment the variable without worry about a concurrency issue.

public void notify(String key) {
    AtomicLong value = stats.get(key);
    if (value == null) {
        value = stats.putIfAbsent(key, new AtomicLong(1));
    }
    if (value != null) {
        value.incrementAndGet();
    }
}

This should be both fast and thread safe

Edit: Refactored sligthly so there is only at most two lookups.