SortedMap headMap() method in Java

The headMap() method of SortedMap interface in Java is used to return a view of the portion of this map whose keys are strictly less than toKey.

  • The map returned by this method is backed by this map, so changes in the returned map are reflected in this map, and vice-versa.
  • The map returned by this method supports all optional map operations that this map supports.

Note: The map returned by this method will throw an IllegalArgumentException if any attempt is made to insert a key outside its range.

Syntax:



SortedMap<K, V> headMap(K toKey)

Where, K is the type of key maintained by this Set and V is the type of values associated with the Key.

Parameters: This function accepts a single parameter toKey which represents high endpoint (exclusive) of the keys in the returned map.

Return Value: It returns a view of the portion of this map whose keys are strictly less than toKey.

Exception:

  • ClassCastException: If the parameter toKey is not compatible with this map’s comparator (or, if the map has no comparator, if toKey does not implement Comparable).
  • NullPointerException: If the parameter toKey is null and this map does not permit null keys.
  • IllegalArgumentException: If this map itself has a restricted range, and toKey lies outside the bounds of the range

Below programs illustrate the above method:

Program 1:

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// A Java program to demonstrate
// working of SortedSet
import java.util.*;
  
public class Main {
    public static void main(String[] args)
    {
        // Create a TreeSet and inserting elements
        SortedMap<Integer, String> mp = new TreeMap<>();
  
        // Adding Element to SortedSet
        mp.put(1, "One");
        mp.put(2, "Two");
        mp.put(3, "Three");
        mp.put(4, "Four");
        mp.put(5, "Five");
  
        // Returning the map with key less than 3
        System.out.print("Last Key in the map is : "
                         + mp.headMap(3));
    }
}

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Output:

Last Key in the map is : {1=One, 2=Two}

Program 2:

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// A Java program to demonstrate
// working of SortedSet
import java.util.*;
  
public class Main {
    public static void main(String[] args)
    {
        // Create a TreeSet and inserting elements
        SortedMap<String, String> mp = new TreeMap<>();
  
        // Adding Element to SortedSet
        mp.put("One", "Geeks");
        mp.put("Two", "For");
        mp.put("Three", "Geeks");
        mp.put("Four", "Code");
        mp.put("Five", "It");
  
        // Returning map with key less than H
        System.out.print("Last Key in the map is : "
                         + mp.headMap("H"));
    }
}

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Output:

Last Key in the map is : {Five=It, Four=Code}

Reference: https://docs.oracle.com/javase/10/docs/api/java/util/SortedMap.html#headMap(K)



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