java.util.TreeMap.headMap() Method

Description

TheheadMap(K toKey) method is used to return a view of the portion of this map whose keys are strictly less than toKey.

Declaration

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

public SortedMap<K,V> headMap(K toKey)

Parameters

toKey − This is the high endpoint (exclusive) of the keys in the returned map.

Return Value

The method call returns a view of the portion of this map whose keys are strictly less than toKey.

Exception

• ClassCastException − This exception is thrown if toKey is not compatible with this map's comparator (or, if the map has no comparator, if toKey does not implement Comparable). Implementations may, but are not required to, throw this exception if toKey cannot be compared to keys currently in the map.

• 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.

Example

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>();
      SortedMap<Integer, String> treemaphead = 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
      treemaphead = treemap.headMap(3);

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

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

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

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