Native lesson simulator
Hash map lookup
Hash the key, jump to a bucket, then compare entries there.
Ada hashes to bucket 2; lookup only scans entries in that bucket.
Flash cards
Review the key moves
What is the main idea behind Java TreeSet?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
import java.util.TreeSet; // Import the TreeSet ___Put the learning moves in the order that makes the concept easiest to apply.
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
Create a TreeSet object called cars that will store strings:
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);
}
}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:
cars.contains("Mazda");Remove an Element
Use remove() to remove an element
cars.remove("Volvo");Remove All Elements
Use clear() to remove all elements
cars.clear();TreeSet Size
Use size() to count how many unique elements are in the set:
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);
}
}
}Output: The numbers will be printed in sorted order (10, 20, 30, 40).
HashSet vs TreeSet
| Feature | HashSet | TreeSet |
|---|---|---|
| Order | No guaranteed order | Sorted (natural order) |
| Duplicates | Not allowed | Not allowed |
| Performance | Faster (no sorting) | Slower (due to sorting) |
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:
// 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.