Java TreeSet

Java TreeSet

A TreeSet is a collection that stores unique elements in sorted order.

It is part of the java.util package and implements the Set interface.

Tip: Unlike HashSet, which has no order, TreeSet keeps its elements sorted automatically.

Create a TreeSet

Example

import java.util.TreeSet; // Import the TreeSet class

TreeSet<String> cars = new TreeSet<>();

Now you can use methods like add(), contains(), and remove() to manage your sorted set of elements.

Add Elements

To add elements to a TreeSet, use the add() method:

Example

import java.util.TreeSet;

public class Main {
  public static void main(String[] args) {
    TreeSet<String> cars = new TreeSet<>();
    cars.add("Volvo");
    cars.add("BMW");
    cars.add("Ford");
    cars.add("BMW");  // Duplicate
    cars.add("Mazda");

    System.out.println(cars);
  }
}

Note: Output: The elements will be sorted automatically (e.g., [BMW, Ford, Mazda, Volvo]). Note: Duplicates like "BMW" will only appear once.

Check if an Element Exists

Use contains() to check if an element exists:

Example

cars.contains("Mazda");

Remove an Element

Use remove() to remove an element:

Example

cars.remove("Volvo");

Remove All Elements

Use clear() to remove all elements:

Example

cars.clear();

TreeSet Size

Use size() to count how many unique elements are in the set:

Example

cars.size();

Note: Duplicate values are not counted - only unique elements are included in the size.

Loop Through a TreeSet

Loop through the elements of a TreeSet with a for-each loop:

Example

TreeSet<String> cars = new TreeSet<>();
// add elements...

for (String i : cars) {
  System.out.println(i);
}

Using TreeSet with Numbers

TreeSet also works with numbers and sorts them from smallest to largest:

Example

import java.util.TreeSet;

public class Main {
  public static void main(String[] args) {
    TreeSet<Integer> numbers = new TreeSet<>();
    numbers.add(40);
    numbers.add(10);
    numbers.add(30);
    numbers.add(20);

    for (int n : numbers) {
      System.out.println(n);
    }
  }
}

Note: Output: The numbers will be printed in sorted order (10, 20, 30, 40).

HashSet vs TreeSet

Tip: Use HashSet when you care about speed, and TreeSet when you need sorted elements.

The var Keyword

From Java 10, you can use the var keyword to declare a TreeSet variable without writing the type twice. The compiler figures out the type from the value you assign.

This makes code shorter, but many developers still use the full type for clarity. Since var is valid Java, you may see it in other code, so it's good to know that it exists:

Example

// Without var
TreeSet<String> cars = new TreeSet<String>();

// With var
var cars = new TreeSet<String>();

The Set Interface

Note: Sometimes you will see both Set and TreeSet in Java code, like this:

import java.util.Set;
import java.util.TreeSet;

Set<String> cars = new TreeSet<>();

This means the variable (cars) is declared as a Set (the interface), but it stores a TreeSet object (the actual set). Since TreeSet implements the Set interface, this is possible.

It works the same way, but some developers prefer this style because it gives them more flexibility to change the type later.