问题:

In a project I need to remove all objects having key value greater than a certain key (key type is Date, if it matters).

As far as I know TreeMap implemented in Java is a red-black tree which is a binary search tree. So I should get O(n) when removing a subtree.
But I can't find any method to do this other than making a tail view and remove one by one, which takes O(logn).

Any good ideas implementing this function? I believe treeMap is the correct dataStructure to use and should be able to do this.

thanks in advance

回答1:

Quite simple. Instead of removing the entries one-by-one, use Map.clear() to remove the elements. In code:

map.tailMap(key).clear();

回答2:

public class TreeMapDemo {
    public static void main(String[] args) {
            // creating maps
            TreeMap<Integer, String> treemap = new TreeMap<Integer, String>();
            SortedMap<Integer, String> treemapincl = new TreeMap<Integer, String>();

            // populating tree map
            treemap.put(2, "two");
            treemap.put(1, "one");
            treemap.put(3, "three");
            treemap.put(6, "six");
            treemap.put(5, "five");

            System.out.println("Getting tail map");
            treemap.tailMap(3).clear();

            System.out.println("Tail map values: " + treemapincl);
            System.out.println("Tree map values: " + treemap);
     }
}

This will remove the elements in the tree map.