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TreeSet headSet() Method in Java With Examples

  • Last Updated : 01 Nov, 2021

TreeSet is one of the most important implementations of the SortedSet interface in Java that uses a Tree for storage. The ordering of the elements is maintained by a set using their natural ordering whether or not an explicit comparator is provided. This must be consistent with equals if it is to correctly implement the Set interface

The headSet() method of TreeSet class been present inside java.util package is used as a limit setter for a tree set, to return the elements up to a limit defined in the parameter of the method in a sorted manner excluding the element.

Syntax: 

head_set = (TreeSet)tree_set.headSet(Object element)

Parameters: The parameter element is of the type of the tree set and is the headpoint that is the limit up to which the tree is allowed to return values excluding the element itself.

Return Values: The method returns the portion of the values in a sorted manner that is strictly less than the element mentioned in the parameter.

now we will be discussing different scenarios while implementing the headSet() method in TreeSet class:

  • Case 1: In a sorted TreeSet
  • Case 2-A: In an unsorted TreeSet
  • Case 2-B: In an unsorted TreeSet but with String type elements

Example 1:

Java




// Java program to Illustrate headSet() method
// of TreeSet class In a sorted TreeSet
  
// Importing required classes
import java.io.*;
import java.util.Iterator;
import java.util.TreeSet;
  
// Main class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Creating an empty TreeSet by
        // declaring object of TreeSet class
        TreeSet<Integer> tree_set = new TreeSet<Integer>();
  
        // Adding the elements
        // using add() method
        tree_set.add(1);
        tree_set.add(2);
        tree_set.add(3);
        tree_set.add(4);
        tree_set.add(5);
        tree_set.add(10);
        tree_set.add(20);
        tree_set.add(30);
        tree_set.add(40);
        tree_set.add(50);
  
        // Creating the headSet tree
        TreeSet<Integer> head_set = new TreeSet<Integer>();
  
        // Limiting the values till 5
        head_set = (TreeSet<Integer>)tree_set.headSet(30);
  
        // Creating an Iterator
        Iterator iterate;
        iterate = head_set.iterator();
  
        // Displaying the tree set data
        System.out.println(
            "The resultant values till head set: ");
  
        // Holds true till there is single element
        // remaining in the object
        while (iterate.hasNext()) {
  
            // Iterating through the headSet
            // using next() method
            System.out.println(iterate.next() + " ");
        }
    }
}
Output: 
The resultant values till head set: 
1 
2 
3 
4 
5 
10 
20

 

Example 2-A:

Java




// Java Program to illustrate headSet() method
// of TreeSet class In an unsorted TreeSet
  
// Importing required classes 
import java.io.*;
import java.util.Iterator;
import java.util.TreeSet;
  
// Main class 
public class GFG {
    
    // Main driver method 
    public static void main(String[] args)
    {
  
        // Creating an empty TreeSet
        TreeSet<Integer> tree_set = new TreeSet<Integer>();
  
        // Adding the elements
        // using add() method 
        tree_set.add(9);
        tree_set.add(2);
        tree_set.add(100);
        tree_set.add(40);
        tree_set.add(50);
        tree_set.add(10);
        tree_set.add(20);
        tree_set.add(30);
        tree_set.add(15);
        tree_set.add(16);
  
        // Creating the headSet tree
        TreeSet<Integer> head_set = new TreeSet<Integer>();
  
        // Limiting the values till 5
        head_set = (TreeSet<Integer>)tree_set.headSet(30);
  
        // Creating an Iterator
        Iterator iterate;
        iterate = head_set.iterator();
  
        // Displaying the tree set data
        System.out.println("The resultant values till head set: ");
  
        // Iterating through the headSet
        while (iterate.hasNext()) {
            
            // Printing the elements 
            System.out.println(iterate.next() + " ");
        }
    }
}
Output: 
The resultant values till head set: 
2 
9 
10 
15 
16 
20

 

Example 2-B:

Java




// Java code to illustrate headSet() method of TreeSet class
// In an unsorted treeset but with String type elements
  
// Importing required classes
import java.io.*;
import java.util.Iterator;
import java.util.TreeSet;
  
// Main class
public class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
  
        // Creating an empty TreeSet
        TreeSet<String> tree_set = new TreeSet<String>();
  
        // Adding the elements using add()
        tree_set.add("Welcome");
        tree_set.add("To");
        tree_set.add("Geek");
        tree_set.add("4");
        tree_set.add("Geeks");
        tree_set.add("TreeSet");
  
        // Creating the headSet tree
        TreeSet<String> head_set = new TreeSet<String>();
  
        // Limiting the values till 5
        head_set = (TreeSet<String>)tree_set.headSet("To");
  
        // Creating an Iterator
        Iterator iterate;
        iterate = head_set.iterator();
  
        // Displaying the tree set data
        System.out.println(
            "The resultant values till head set: ");
  
        // Iterating through the headSet
        while (iterate.hasNext()) {
  
            // Printing elements using next() method
            System.out.println(iterate.next() + " ");
        }
    }
}
Output: 
The resultant values till head set: 
4 
Geek 
Geeks

 


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