SortedSet headSet() method in Java

The headSet() method of SortedSet interface in Java is used to return a view of the portion of this set whose elements are strictly less than the parameter toElement.

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

Note: The set returned by this method will throw an IllegalArgumentException if an attempt is made to insert an element outside its range.

Syntax:

SortedSet headSet(E toElement)

Where, E is the type of element maintained by this Set.

Parameters: This function accepts a single parameter toElement which represent the high endpoint (exclusive) of the returned set.

Return Value: It returns the last or the highest element currently in the set.

Exceptions:

• ClassCastException : It throws a ClassCastException if toElement is not compatible with this set’s comparator (or, if the set has no comparator, if toElement does not implement Comparable).
• NullPointerException : It throws a NullPointerException if the parameter toElement is null.
• IllegalArgumentException : It throws an IllegalArgumentException this set itself has a restricted range, and the parameter toElement 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.SortedSet;
import java.util.TreeSet;

public class Main {
    public static void main(String[] args)
    {
        // Create a TreeSet and inserting elements
        SortedSet<Integer> s = new TreeSet<>();

        // Adding Element to SortedSet
        s.add(1);
        s.add(5);
        s.add(2);
        s.add(3);
        s.add(9);

        // Returning the set with elements
        // strictly less than the passed value
        System.out.print("Elements strictly less than 7 in set are : "
                         + s.headSet(7));
    }
}

Output:

Elements strictly less than 7 in set are : [1, 2, 3, 5]

Program 2:

// A Java program to demonstrate
// working of SortedSet
import java.util.SortedSet;
import java.util.TreeSet;

public class Main {
    public static void main(String[] args)
    {
        // Create a TreeSet and inserting elements
        SortedSet<String> s = new TreeSet<>();

        // Adding Element to SortedSet
        s.add("Geeks");
        s.add("For");
        s.add("Geeks");
        s.add("Code");
        s.add("It");

        // Returning the set with elements
        // strictly less than the passed value
        System.out.print("Element strictly less than Z in set is : "
                         + s.headSet("Z"));
    }
}

Output:

Element strictly less than Z in set is : [Code, For, Geeks, It]

Reference: https://docs.oracle.com/javase/10/docs/api/java/util/SortedSet.html#headSet(E)