Java TreeSet

TreeSet is a Set implementation that stores its elements in a self-balancing tree, keeping them automatically sorted in ascending order at all times.

In this page:

Introduction to TreeSet
Automatic Sorting
Adding and Removing Elements
Navigation Methods
Custom Sort Order with Comparator

Syntax

java

TreeSet<Type> set = new TreeSet<>();
set.add(item);  // sorted order
set.first();

Introduction to TreeSet

TreeSet is an implementation of the Set interface that stores its elements in a red-black tree, a self-balancing binary search tree, which keeps every element automatically arranged in sorted order at all times.

Example: Introduction to TreeSet

java

import java.util.TreeSet;
public class Main {
	public static void main(String[] args) {
		TreeSet<Integer> set = new TreeSet<>(); // red-black tree
		set.add(5);
		set.add(1);
		System.out.println(set);
	}
}

Automatic Sorting

Elements added to a TreeSet are sorted according to their natural ordering by default -- numbers sort numerically and strings sort alphabetically -- regardless of the order they happened to be inserted in.

Example: Automatic Sorting

java

import java.util.TreeSet;
public class Main {
	public static void main(String[] args) {
		TreeSet<Integer> set = new TreeSet<>();
		set.add(3);
		set.add(1);
		set.add(2);
		System.out.println(set); // sorted: 1, 2, 3
	}
}

Adding and Removing Elements

TreeSet supports the same add and remove operations as any other Set, but every insertion or removal also re-balances the underlying tree to keep the sorted order intact, and duplicate values are still rejected.

Example: Adding and Removing Elements

java

import java.util.TreeSet;
public class Main {
	public static void main(String[] args) {
		TreeSet<Integer> set = new TreeSet<>();
		set.add(5);
		set.add(5); // duplicate ignored
		set.remove(5);
		System.out.println(set);
	}
}

Navigation Methods

TreeSet adds navigation methods beyond a plain Set, like first, last, higher, and lower, which take advantage of the guaranteed sort order to efficiently find the smallest, largest, or nearest elements relative to a given value.

Example: Navigation Methods

java

// Import java.util.TreeSet so it can be used by its short name
import java.util.TreeSet;
// Define the class Main
public class Main {
	// Program entry point: the JVM starts running here
	public static void main(String[] args) {
		// Create a new TreeSet object and store it in set
		TreeSet<Integer> set = new TreeSet<>();
		set.add(1); set.add(5); set.add(10);
		// Print a line to the console
		System.out.println(set.first());
		// Print a line to the console
		System.out.println(set.last());
		System.out.println(set.higher(5));
	}
}

Custom Sort Order with Comparator

Passing a Comparator to a TreeSet's constructor overrides the default natural ordering, letting elements be sorted by any custom rule, such as reverse order or by a computed property like string length instead of alphabetical order.

Example: Custom Sort Order with Comparator

java

import java.util.TreeSet;
import java.util.Collections;
public class Main {
	public static void main(String[] args) {
		TreeSet<Integer> set = new TreeSet<>(Collections.reverseOrder());
		set.add(1);
		set.add(5);
		System.out.println(set); // reverse sorted
	}
}

Chapter Quiz — Complete all 17 topics to unlock

0/17 topics done

Complete these topics first:

← Previous Lesson Java HashSet Next Lesson → Java LinkedHashSet

Java TreeSet

Introduction to TreeSet

Automatic Sorting

Adding and Removing Elements

Navigation Methods

Custom Sort Order with Comparator

Chapter Quiz — Complete all 17 topics to unlock

Login to run this code