Map of AtomicInteger

Question

I want to implement a shared object for calculation of statistics of executions of operations. Objects state will be represented by Map<String,AtomicInteger>(key is the name of operation, value is the number of times the operation was executed). Am I correct that I can choose a HashMap<String,AtomicInteger> implementation and use no synchronization on it for getting values from it since AtomicInteger has a volatile value field underneath it.

Sample of code that does addition and incrementation of execution stats:

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.atomic.AtomicInteger;

public final class Stats {

private final Map<String, AtomicInteger> statistics = new HashMap<String, AtomicInteger>();

public int increment(String operationName) {
    if (!statistics.containsKey(operationName)) {
        synchronized (statistics) {
            if (!statistics.containsKey(operationName)) 
                statistics.put(operationName, new AtomicInteger(0));
        }
    }

    return statistics.get(operationName).getAndIncrement();
}

public int getOpStats(String operationName) {
    if (!statistics.containsKey(operationName)) {
        return 0;
    }
    return statistics.get(operationName).get();
}

}

Answer 1

If you want to be thread safe with regards to counter initialization you should use a ConcurrentHashMap and always instanciate-and-increase the counters this way:

themap.putIfAbsent("the name", new AtomicInteger(0)); 
themap.get("the name").incrementAndGet();

You could also make sure you initialize all the counters used before you start, and just use whatever collection you like. A plain AtomicInteger[]-array is by far quickest, given that you know where to look, HashTable could be slightly quicker than HashMap.

If you know on beforehand which counters you have, you could also define a java enum of all the counter names and use an EnumMap<YourCountersEnum, AtomicInteger>. This would probably give look-up performance close to an AtomicInteger[]-array lookup.