This section provides code and benchmarks for ten unique example implementations which iterate over the entries of a Map<Integer, Integer> and generate the sum of the Integer values. All of the examples have an algorithmic complexity of Θ(n), however, the benchmarks are still useful for providing insight on which implementations are more efficient in a “real world” environment.

1. Implementation using Iterator with Map.Entry

Iterator<Map.Entry<Integer, Integer>> it = map.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry<Integer, Integer> pair = it.next();
    sum += pair.getKey() + pair.getValue();
}

1. Implementation using for with Map.Entry

for (Map.Entry<Integer, Integer> pair : map.entrySet()) {
    sum += pair.getKey() + pair.getValue();
}

1. Implementation using Map.forEach (Java 8+)

map.forEach((k, v) -> sum[0] += k + v);

1. Implementation using Map.keySet with for

for (Integer key : map.keySet()) {
    sum += key + map.get(key);
}

1. Implementation using Map.keySet with Iterator

Iterator<Integer> it = map.keySet().iterator();
while (it.hasNext()) {
    Integer key = it.next();
    sum += key + map.get(key);
}

1. Implementation using for with Iterator and Map.Entry

for (Iterator<Map.Entry<Integer, Integer>> entries = 
         map.entrySet().iterator(); entries.hasNext(); ) {
    Map.Entry<Integer, Integer> entry = entries.next();
    sum += entry.getKey() + entry.getValue();
}

1. Implementation using Stream.forEach (Java 8+)

map.entrySet().stream().forEach(e -> sum += e.getKey() + e.getValue());

1. Implementation using Stream.forEach with Stream.parallel (Java 8+)

map.entrySet()
   .stream()
   .parallel()
   .forEach(e -> sum += e.getKey() + e.getValue());

1. Implementation using IterableMap from Apache Collections

MapIterator<Integer, Integer> mit = iterableMap.mapIterator();
while (mit.hasNext()) {
    sum += mit.next() + it.getValue();
}

1. Implementation using MutableMap from Eclipse Collections