Performance considerations for keySet() and entrySet() of Map

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The most common case where using entrySet is preferable over keySet is when you are iterating through all of the key/value pairs in a Map.

This is more efficient:

for (Map.Entry entry : map.entrySet()) {
    Object key = entry.getKey();
    Object value = entry.getValue();
}

than:

for (Object key : map.keySet()) {
    Object value = map.get(key);
}

Because in the second case, for every key in the keySet the map.get() method is called, which – in the case of a HashMap – requires that the hashCode() and equals() methods of the key object be evaluated in order to find the associated value*. In the first case that extra work is eliminated.

Edit: This is even worse if you consider a TreeMap, where a call to get is O(log2(n)), i.e. the comparator for will may need to run log2(n) times (n = size of the Map) before finding the associated value.

*Some Map implementations have internal optimisations that check the objects’ identity before the hashCode() and equals() are called.

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