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

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;
public class Main {
	public static void main(String[] args) {
		TreeSet<Integer> set = new TreeSet<>();
		set.add(1); set.add(5); set.add(10);
		System.out.println(set.first());
		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
	}
}

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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

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