TreeSet headSet() Method in Java With Examples - GeeksforGeeks

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 classesimport java.io.*;import java.util.Iterator;import java.util.TreeSet;  // Main classpublic 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() + " ");        }    }}
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() + " ");        }    }}
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 classesimport java.io.*;import java.util.Iterator;import java.util.TreeSet;  // Main classpublic 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() + " ");        }    }}
The resultant values till head set: 
4 
Geek 
Geeks
 
solankimayank
Java - util package
Java-Collections
java-treeset
Java
Java
Java-Collections
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Comments
Old Comments
Constructors in Java
Stream In Java
Exceptions in Java
Functional Interfaces in Java
Different ways of Reading a text file in Java
Java Programming Examples
Internal Working of HashMap in Java
Checked vs Unchecked Exceptions in Java
Strings in Java
StringBuilder Class in Java with Examples