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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:

Java

// 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:

Java

// 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)

SortedMap headMap() method in Java

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