+ public synchronized void addToSum(String user, Integer value)
+ {
+ if (value == null || value < 1)
+ throw new IllegalArgumentException("Not a positive number: " + value);
+
+ long sum =
+ Optional
+ .ofNullable(state.get(user))
+ .map(result -> result.sum)
+ .orElse(0l);
+ state.put(user, new AdderResult(value, sum + value));