How select first N items in Java TreeMap?

Question

Given this map

SortedMap<Integer, String> myMap = new TreeMap<Integer, String>();

Instead of a for loop is there a utility function to copy first N items to a destination map?

Answer 1

Using the power of Java 8+:

TreeMap<Integer, String> myNewMap = myMap.entrySet().stream()
    .limit(3)
    .collect(TreeMap::new, (m, e) -> m.put(e.getKey(), e.getValue()), Map::putAll);

Answer 2

Maybe, but not as part of the standard Java API. And: the utility would use a loop inside.

So you'll need a loop, but you can create your own "utility" by doing it all in a static method in a utility class:

public static SortedMap<K,V> putFirstEntries(int max, SortedMap<K,V> source) {
  int count = 0;
  TreeMap<K,V> target = new TreeMap<K,V>();
  for (Map.Entry<K,V> entry:source.entrySet()) {
     if (count >= max) break;

     target.put(entry.getKey(), entry.getValue());
     count++;
  }
  return target;
}

The complexity is still O(n) (I doubt, that one can achieve O(1)) but you use it like a tool without "seeing" the loop:

SortedMap<Integer, String> firstFive = Util.putFirstEntries(5, sourceMap);