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javacollections

Iterating HashMap in order

I have a HashMap.

It has 100s of millions of observations.

What's the best way to iterate over the elements of the HashMap, in numerical order of the keys?

I considered changing to TreeMap, but did not do that since it may actually increase the load in creating the Map (as TreeMap is O(n), HashMap is O(1)).

Solution

  • With Java 8 you could use something similar to the following:

    import static java.util.Comparator.comparing;

map.entrySet().stream()
   .sorted(comparing(Entry::getKey))
   .forEach(e -> doSomethingWithTheEntry(e));

    That will obviously involve sorting the unsorted keys, which will come at a cost. So you need to decide whether you want to pay the cost upfront with a TreeMap or when required and keep using a HashMap.