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:

// 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));
    }
}

Output:

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

Program 2:

// 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"));
    }
}

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)