TreeSet Class in Java

Last Updated: 03 October, 2023

The TreeSet class of the Java collections framework was introduced in the JDK 1.2 version and is available in the java.util package.

A TreeSet is a sorted collection of objects. A TreeSet is the implementation 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 (ascending) order.

TreeSet provides fast access and retrieval operations, which makes TreeSet an excellent choice when storing large amounts of sorted information that must be found quickly.

👉 Important features of Java TreeSet:

• The underlying data structure of the TreeSet is a balanced tree.
• TreeSet stores elements in a default ascending order.
• TreeSet class internally uses a TreeMap to store elements.
• TreeSet only contains unique elements; it does not accept duplicates.
• TreeSet does not accept duplicate or null elements.
• TreeSet is, by default, a non-synchronized class.
• Access and retrieval times are quite fast, which makes TreeSet an excellent choice when storing large amounts of sorted information.

👉 Java TreeSet Class Hierarchy

As we can see from the complete hierarchy above, TreeSet extends the AbstractSet class and implements the NavigableSet interface as well as the Serializable and Cloneable interfaces.

👉 Java TreeSet Class Declaration

Here, E defines the type of elements that the Set will contain.

👉 Example: A sample Java program of TreeSet

import java.util.TreeSet; public class TreeSetExample1 { public static void main(String[] args) { // Creating a TreeSet TreeSet<Integer> tree = new TreeSet<Integer>(); // Adding elements in TreeSet tree.add(40); tree.add(20); tree.add(10); tree.add(50); tree.add(30); tree.add(30); // Printing TreeSet System.out.println(tree); } }

Output

[10, 20, 30, 40, 50]

👉 Java TreeSet Class Constructors

The Java TreeSet class has the following constructors:

👉 Java TreeSet Class Methods

The Java TreeSet class has the following methods:

TreeSet Class Methods Examples

The Java TreeSet class provided methods are implemented in the below given examples. Let's go through the examples one by one and understand the uses for each method.

Example 1 : Methods for Adding elements in TreeSet

import java.util.Arrays; import java.util.TreeSet; public class AddElementsInTreeSet { public static void main(String[] args) { /****************************************** * TreeSet - add(), addAll() Methods Example *******************************************/ // Creating a TreeSet TreeSet<Integer> treeSet = new TreeSet<Integer>(); // Adding elements into the TreeSet treeSet.add(800); treeSet.add(500); treeSet.add(200); treeSet.add(500); // Adding duplicate element treeSet.add(400); // treeSet.add(null); // Throws NullPointerException at run time // Printing TreeSet elements System.out.println("TreeSet elements: " + treeSet); // Creating another TreeSet TreeSet<Integer> childTree = new TreeSet<Integer>(Arrays.asList(10, 20, 30, 40)); // Adding TreeSet into another TreeSet treeSet.addAll(childTree); // Printing TreeSet elements System.out.println("\nTreeSet elements after adding another TreeSet"); System.out.println(treeSet); } }

Output

TreeSet elements: [200, 400, 500, 800]

TreeSet elements after adding another TreeSet
[10, 20, 30, 40, 200, 400, 500, 800]

Example 2 : Methods for Accessing elements from TreeSet

import java.util.Arrays; import java.util.TreeSet; public class AccessElementsFromTressSet { public static void main(String[] args) { /******************************************************************************** TreeSet - ceiling(), floor(), pollFirst(), pollLast(), first(), last() Methods ********************************************************************************/ // Creating and adding elements into HashSet TreeSet<Integer> treeSet = new TreeSet<Integer>(Arrays.asList(80, 30, 10, 50, 20, 60, 40)); System.out.println("TreeSet elements: " + treeSet); // Returns the least element in this set greater than or equal to the given // element, or null if there is no such element. System.out.println("treeSet.ceiling(30) : " + treeSet.ceiling(30)); // Returns the greatest element in this set less than or equal to the given // element, or null if there is no such element. System.out.println("treeSet.floor(40) : " + treeSet.floor(40)); // Retrieves and removes the first (lowest) element. System.out.println("treeSet.pollFirst() : " + treeSet.pollFirst()); // Retrieves and removes the last (highest) element. System.out.println("treeSet.pollLast() : " + treeSet.pollLast()); // Print TreeSet in reverse order System.out.println("treeSet.descendingSet() : " + treeSet.descendingSet()); // Returns the first (lowest) element currently in this set. System.out.println("treeSet.first() : " + treeSet.first()); // Returns the last (highest) element currently in this set System.out.println("treeSet.last() : " + treeSet.last()); } }

Output

TreeSet elements: [10, 20, 30, 40, 50, 60, 80]
treeSet.ceiling(30) : 30
treeSet.floor(40) : 40
treeSet.pollFirst() : 10
treeSet.pollLast() : 80
treeSet.descendingSet() : [60, 50, 40, 30, 20]
treeSet.first() : 20
treeSet.last() : 60

Example 3 : Methods for Removing elements from TreeSet

import java.util.Arrays; import java.util.TreeSet; public class RemoveElementsFromTreeSet { public static void main(String[] args) { /************************************************************************ TreeSet - remove(), removeAll(), removeIf(), retainAll() Methods Example *************************************************************************/ // Creating and adding elements into TreeSet TreeSet<Integer> treeSet = new TreeSet<Integer>(Arrays.asList(10, 20, 30, 40, 50, 60, 70)); System.out.println("TreeSet elements: " + treeSet); // Removing the specified element from TreeSet treeSet.remove(40); System.out.println("After calling remove() method: " + treeSet); // Creating another TreeSet and adding elements TreeSet<Integer> treeSetSecond = new TreeSet<Integer>(Arrays.asList(20, 60, 70)); // Removing all those elements that matches with second TreeSet elements treeSet.removeAll(treeSetSecond); System.out.println("After calling removeAll() method: " + treeSet); // Removing elements on the basis of specified condition treeSet.removeIf(i -> i == 10); System.out.println("After calling removeIf() method: "+ treeSet); // Removes from the TreeSet all of its elements that are not contained in the specified collection treeSet.retainAll(treeSetSecond); System.out.println("After calling retainingAll() method: " + treeSet); // Removing all the elements from the TreeSet treeSet.addAll(treeSetSecond); System.out.println("Before calling clear() method: " + treeSet); treeSet.clear(); System.out.println("After calling clear() method: " + treeSet); } }

Output

TreeSet elements: [10, 20, 30, 40, 50, 60, 70]
After calling remove() method: [10, 20, 30, 50, 60, 70]
After calling removeAll() method: [10, 30, 50]
After calling removeIf() method: [30, 50]
After calling retainingAll() method: []
Before calling clear() method: [20, 60, 70]
After calling clear() method: []

Example 4 : Utility methods of TreeSet

import java.util.ArrayList; import java.util.Arrays; import java.util.TreeSet; public class TreeSetUtilityMethods { public static void main(String[] args) { // Creating and adding elements into TreeSet TreeSet<Integer> treeSet = new TreeSet<Integer>(Arrays.asList(50, 30, 70, 10, 40, 20, 80)); System.out.println("TreeSet elements: " + treeSet); // Printing TreeSet Size System.out.println("TreeSet Size: " + treeSet.size()); // Returns true if the TreeSet contains no elements. System.out.println("Is TreeSet Empty: " + treeSet.isEmpty()); // Returns true if the TreeSet contains the specified element System.out.println("TreeSet contains() method: " + treeSet.contains(40)); // Creating another TreeSet and adding elements TreeSet<Integer> treeSetSecond = new TreeSet<Integer>(Arrays.asList(20, 60, 70)); // Returns true if the TreeSet contains all of the elements of the specified collection System.out.println("TreeSet containsAll() method: " + treeSet.containsAll(treeSetSecond)); // Converting TreeSet into ArrayList ArrayList<Integer> arrayList = new ArrayList<Integer>(treeSet); System.out.println("ArrayList elements: " + arrayList); } }

Output

TreeSet elements: [10, 20, 30, 40, 50, 70, 80]
TreeSet Size: 7
Is TreeSet Empty: false
TreeSet contains() method: true
TreeSet containsAll() method: false
ArrayList elements: [10, 20, 30, 40, 50, 70, 80]

Reference: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/TreeSet.html