NavigableSet headSet() method in Java

The headSet() method of

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.

Java

// 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.

Java

// 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)

NavigableSet headSet() method in Java

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