NavigableSet headSet() method in Java

The headSet() method of NavigableSet interface in Java is used to return a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement.

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

Note: The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.

Syntax:

NavigableSet<E> headSet(E toElement,
                        boolean inclusive)

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

Parameters: This function has two parameters :

• toElement – high endpoint of the returned set
• inclusive – true if the high endpoint is to be included in the returned view

Return Value: It return a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement.

Below programs illustrate the headSet() method in Java:

Program 1: NavigableSet with integer elements.

// A Java program to demonstrate
// headSet() method of NavigableSet

import java.util.NavigableSet;
import java.util.TreeSet;

public class GFG {
    public static void main(String[] args)
    {
        NavigableSet<Integer> ns = new TreeSet<>();
        ns.add(0);
        ns.add(1);
        ns.add(2);
        ns.add(3);
        ns.add(4);
        ns.add(5);
        ns.add(6);

        System.out.println("Map with key-value less than "+
                        "given argument : " + ns.headSet(6));

        System.out.println("Map with key-value less than or" +
          " equal to  given argument : " + ns.headSet(6, true));
    }
}

Output:

Map with key-value less than given argument : [0, 1, 2, 3, 4, 5]
Map with key-value less than or equal to  given argument : [0, 1, 2, 3, 4, 5, 6]

Program 2: NavigableSet with string elements.

// A Java program to demonstrate
// headSet?() method of NavigableSet

import java.util.NavigableSet;
import java.util.TreeSet;

public class GFG {
    public static void main(String[] args)
    {
        NavigableSet<String> ns = new TreeSet<>();
        ns.add("A");
        ns.add("B");
        ns.add("C");
        ns.add("D");
        ns.add("E");
        ns.add("F");
        ns.add("G");

        System.out.println("Map with key-value less than"+
                    " given argument : " + ns.headSet("F"));

        System.out.println("Map with key-value less than "+
           "or equal to given argument : " + ns.headSet("F"));
    }
}

Output:

Map with key-value less than given argument : [A, B, C, D, E]
Map with key-value less than or equal to given argument : [A, B, C, D, E]

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