synchronize a method by achieving better performance?

flash :

I have a class that is being called by multiple threads on multi core machine. I want to make it thread safe.

add method will be called by multiple threads. And if key exists, just append the current value to new value otherwise just put key and value in the map.

Now to make it thread safe, I was planning to synchronize add method but it will destroy performance. Is there any better way by which we can achieve better performance without synchronizing add method?

class Test {
  private final Map<Integer, Integer> map = new ConcurrentHashMap<>();

  public void add(int key, int value) {
    if (map.containsKey(key)) {
      int val = map.get(key);
      map.put(key, val + value);
      return;
    }
    map.put(key, value);
  }

  public Object getResult() {
    return map.toString();
  }
}

Not a JD :

Use merge:

class Test {
    final Map<Integer, Integer> map = new ConcurrentHashMap<>();

    public void add(int key, int value) {
        map.merge(key, value, Integer::sum);
    }

    public Object getResult() {
        return map.toString();
    }
}

Java 7 solution if you absolutely can't use synchronized (or, you absolutely cannot lock explicitly):

class Test {
    final Map<Integer, AtomicInteger> map = new ConcurrentHashMap<>();

    public void add(int key, int value) {
        get(key).addAndGet(value);
    }

    private AtomicInteger get(int key) {
        AtomicInteger current = map.get(key);

        if (current == null) {
            AtomicInteger ai = new AtomicInteger();

            current = map.putIfAbsent(key, ai);

            if (current == null) {
                current = ai;
            }
        }

        return current;
    }

    public Object getResult() {
        return map.toString();
    }
}

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