获取TreeMap的键集的子集的简单方法（使用Java）

Question

Is it possible to extract the top n (I use top because I believe a TreeMap is sorted) key elements from a TreeMap, without iterating using an Iterator object.

I can do the iteration but it's tedious to have to check for nulls etc.

Answer 1

Why would you be checking for nulls?

If you use the Google Collections Library you can use Iterables.limit .

EDIT: For some reason the GCL page doesn't include limit , but it's in Guava (which has effectively replaced the GCL) - it's still Iterables.limit .

Answer 2

You can easily get the subset of keys from one key up to another using .subMap(low,high).keySet() or .headMap(high).keySet() .

Finding out the correct high key for the n th is harder, as there is no direct approach and iterating is the only sure-fire way.

(this code is untested )

public <K,V> SortedMap<K,V> subMap(SortedMap<K,V> map, int n) {
  Iterator<K> it = map.keySet().iterator();
  for (int i = 0; i<n && it.hasNext(); i++) {
    it.next();
  }
  if (it.hasNext()) {
    return map.headMap(it.next());
  } else {
    return map
  }
}

Answer 3

you can use:

[subMap method][1]

public NavigableMap<K,V> subMap(K fromKey,boolean fromInclusive,K toKey,boolean toInclusive)

[1]: http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html#subMap(K , boolean, K, boolean)