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:

Java

// 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:

Java

// 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)

SortedSet headSet() method in Java

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