What is the TreeSet.headSet() function in Java?

The TreeSet.headSet() method is present in the TreeSet class in the java.util package. It is used to return the elements up to a given limit.

The following is the method prototype:

TreeSet head_set.headSet(Object element)

Parameters

The TreeSet.headSet() method takes the following parameter:

Limit_element: The limit up to which TreeSet is allowed to return a value excluding the limit itself.

Return value

Elements: It returns the elements of the TreeSet up to the limit, excluding the Limit_element, in a sorted manner.

Example

Let’s understand with the help of an example.

We have a TreeSet: [1, 3, 5, 8, 9, 12, 15, 23, 25]

It has a limit: 15.

The headset should be 1 3 5 8 9 12.

So, the result of the TreeSet.headSet() method is 1 3 5 8 9 12.

Code

Let’s have a look at the code snippet.

import java.io.*;
import java.util.*;
class Main 
{
    public static void main(String args[])
    {
       TreeSet<Integer> tree_set = new TreeSet<Integer>();
       tree_set.add(1);
       tree_set.add(8);
       tree_set.add(5);
       tree_set.add(3);
       tree_set.add(0);
       tree_set.add(22);
       tree_set.add(10);
       System.out.println("TreeSet: " + tree_set);
       TreeSet<Integer> headset = new TreeSet<Integer>();
       headset = (TreeSet<Integer>)tree_set.headSet(10);
  
       Iterator i;
       i=headset.iterator();
       System.out.print("The resultant elements of the headset are: ");
            while (i.hasNext()) 
       System.out.print(i.next() + " ");
    }
}

Explanation

In line 8, we declared a TreeSet of Integer type.
In lines 9 to 15, we added the elements into the TreeSet by using the TreeSet.add() method.
In line 16, we displayed the original TreeSet with a message.
In line 18, we declared another TreeSet to store the headset.
In line 19, we called the TreeSet.headSet() method with a limit and stored the headset in another TreeSet declared in line 18.
In line 21, we made an object of the Iterator class.
In line 22, we iterated and assigned the TreeSet containing the headset to the Iterator object.
In line 24, we displayed the message regarding the headset.
In lines 25 and 26, we used a while loop to traverse over the headset using the Iterator object and display the elements of the headset.

In this way, we can use the TreeSet.headSet() method to obtain the headset within a limit of the TreeSet.

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