Performance: Creating an ArrayList from HashMap.values()

Question

Question is how much it costs to create an ArrayList from a HashMap.values() Collection? Or creating the values Collection alone? Assuming Map.size() > 100k. Objects could also

Answer 1

HashMap internally stores values in a Collection values. Take a look at the source code of AbstractMap, the parent of HashMap.

So HashMap.values() directly returns a Collection. There is no computation or data copying done. It's as fast as it can be.

Just get the values and then do a for loop:

int n = 5;  // every 5th element
Object[] values = hashMap.values().toArray();
int size = values.length;
for (int i = 0; i < size; i += n){
   values[i];
   // do something
)