java.util.TreeMap.headMap() Method


TheheadMap(K toKey,boolean inclusive) method is used to return a view of the portion of this map whose keys are less than (or equal to, if inclusive is true)toKey.


Following is the declaration for java.util.TreeMap.headMap() method.

public NavigableMap<K,V> headMap(K toKey,boolean inclusive)


  • toKey − This is the high endpoint of the keys in the returned map.

  • inclusive − This is true if the high endpoint is to be included in the returned view.

Return Value

The method call returns a view of the portion of this map whose keys are less than (or equal to, if inclusive is true) toKey.


  • ClassCastException − This exception is thrown if toKey is not compatible with this map's comparator.

  • NullPointerException − This exception is thrown if toKey is null and this map uses natural ordering, or its comparator does not permit null keys.

  • IllegalArgumentException − This exception is thrown if this map itself has a restricted range, and toKey lies outside the bounds of the range.


The following example shows the usage of java.util.TreeMap.headMap() method.

package com.tutorialspoint;

import java.util.*;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating maps 
      TreeMap<Integer, String> treemap = new TreeMap<Integer, String>();
      NavigableMap<Integer, String> treemapheadincl = 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");

      // getting head map inclusive 3
      treemapheadincl = treemap.headMap(3,true);

      System.out.println("Checking values of the map");
      System.out.println("Value is: "+ treemapheadincl);

Let us compile and run the above program, this will produce the following result.

Checking values of the map
Value is: {1=one, 2=two, 3=three}
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