Key existence check in HashMap

Question

Is checking for key existence in HashMap always necessary?

I have a HashMap with say a 1000 entries and I am looking at improving the efficiency. If the HashMap is b

Answer 1

I usually use the idiom

Object value = map.get(key);
if (value == null) {
    value = createValue(key);
    map.put(key, value);
}

This means you only hit the map twice if the key is missing