Java TreeSet

The TreeSet class of the Java collections framework provides the functionality of a tree data structure.

It extends the NavigableSet interface.

Creating a TreeSet

In order to create a tree set, we must import the java.util.TreeSet package first.

Once we import the package, here is how we can create a TreeSet in Java.

TreeSet<Integer> numbers = new TreeSet<>();

Here, we have created a TreeSet without any arguments. In this case, the elements in TreeSet are sorted naturally (ascending order).

However, we can customize the sorting of elements by using the Comparator interface. We will learn about it later in this tutorial.

Methods of TreeSet

The TreeSet class provides various methods that allow us to perform various operations on the set.

Insert Elements to TreeSet

• add() - inserts the specified element to the set
• addAll() - inserts all the elements of the specified collection to the set

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {

        TreeSet<Integer> evenNumbers = new TreeSet<>();

        // Using the add() method
        evenNumbers.add(2);
        evenNumbers.add(4);
        evenNumbers.add(6);
        System.out.println("TreeSet: " + evenNumbers);

        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(1);

        // Using the addAll() method
        numbers.addAll(evenNumbers);
        System.out.println("New TreeSet: " + numbers);
    }
}

Output

TreeSet: [2, 4, 6]
New TreeSet: [1, 2, 4, 6]

Access TreeSet Elements

To access the elements of a tree set, we can use the iterator() method. In order to use this method, we must import java.util.Iterator package. For example,

import java.util.TreeSet;
import java.util.Iterator;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Calling iterator() method
        Iterator<Integer> iterate = numbers.iterator();
        System.out.print("TreeSet using Iterator: ");
        // Accessing elements
        while(iterate.hasNext()) {
            System.out.print(iterate.next());
            System.out.print(", ");
        }
    }
}

Output

TreeSet: [2, 5, 6]
TreeSet using Iterator: 2, 5, 6,

Remove Elements

• remove() - removes the specified element from the set
• removeAll() - removes all the elements from the set

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Using the remove() method
        boolean value1 = numbers.remove(5);
        System.out.println("Is 5 removed? " + value1);

        // Using the removeAll() method
        boolean value2 = numbers.removeAll(numbers);
        System.out.println("Are all elements removed? " + value2);
    }
}

Output

TreeSet: [2, 5, 6]
Is 5 removed? true
Are all elements removed? true

Methods for Navigation

Since the TreeSet class implements NavigableSet, it provides various methods to navigate over the elements of the tree set.

1. first() and last() Methods

• first() - returns the first element of the set
• last() - returns the last element of the set

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Using the first() method
        int first = numbers.first();
        System.out.println("First Number: " + first);

        // Using the last() method
        int last = numbers.last();
        System.out.println("Last Number: " + last);
    }
}

Output

TreeSet: [2, 5, 6]
First Number: 2
Last Number: 6

2. ceiling(), floor(), higher() and lower() Methods

• higher(element) - Returns the lowest element among those elements that are greater than the specified element.
• lower(element) - Returns the greatest element among those elements that are less than the specified element.
• ceiling(element) - Returns the lowest element among those elements that are greater than the specified element. If the element passed exists in a tree set, it returns the element passed as an argument.
• floor(element) - Returns the greatest element among those elements that are less than the specified element. If the element passed exists in a tree set, it returns the element passed as an argument.

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(4);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Using higher()
        System.out.println("Using higher: " + numbers.higher(4));

        // Using lower()
        System.out.println("Using lower: " + numbers.lower(4));

        // Using ceiling()
        System.out.println("Using ceiling: " + numbers.ceiling(4));

        // Using floor()
        System.out.println("Using floor: " + numbers.floor(3));

    }
}

Output

TreeSet: [2, 4, 5, 6]
Using higher: 5
Using lower: 2
Using ceiling: 4
Using floor: 2

3. pollfirst() and pollLast() Methods

• pollFirst() - returns and removes the first element from the set
• pollLast() - returns and removes the last element from the set

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(4);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Using pollFirst()
        System.out.println("Removed First Element: " + numbers.pollFirst());

        // Using pollLast()
        System.out.println("Removed Last Element: " + numbers.pollLast());

        System.out.println("New TreeSet: " + numbers);
    }
}

Output

TreeSet: [2, 4, 5, 6]
Removed First Element: 2
Removed Last Element: 6
New TreeSet: [4, 5]

4. headSet(), tailSet() and subSet() Methods

headSet(element, booleanValue)

The headSet() method returns all the elements of a tree set before the specified element (which is passed as an argument).

The booleanValue parameter is optional. Its default value is false.

If true is passed as a booleanValue, the method returns all the elements before the specified element including the specified element.

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(4);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Using headSet() with default boolean value
        System.out.println("Using headSet without boolean value: " + numbers.headSet(5));

        // Using headSet() with specified boolean value
        System.out.println("Using headSet with boolean value: " + numbers.headSet(5, true));
    }
}

Output

TreeSet: [2, 4, 5, 6]
Using headSet without boolean value: [2, 4]
Using headSet with boolean value: [2, 4, 5]

tailSet(element, booleanValue)

The tailSet() method returns all the elements of a tree set after the specified element (which is passed as a parameter) including the specified element.

The booleanValue parameter is optional. Its default value is true.

If false is passed as a booleanValue, the method returns all the elements after the specified element without including the specified element.

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(4);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Using tailSet() with default boolean value
        System.out.println("Using tailSet without boolean value: " + numbers.tailSet(4));

        // Using tailSet() with specified boolean value
        System.out.println("Using tailSet with boolean value: " + numbers.tailSet(4, false));
    }
}

Output

TreeSet: [2, 4, 5, 6]
Using tailSet without boolean value: [4, 5, 6]
Using tailSet with boolean value: [5, 6]

subSet(e1, bv1, e2, bv2)

The subSet() method returns all the elements between e1 and e2 including e1.

The bv1 and bv2 are optional parameters. The default value of bv1 is true, and the default value of bv2 is false.

If false is passed as bv1, the method returns all the elements between e1 and e2 without including e1.

If true is passed as bv2, the method returns all the elements between e1 and e2, including e1.

For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(2);
        numbers.add(5);
        numbers.add(4);
        numbers.add(6);
        System.out.println("TreeSet: " + numbers);

        // Using subSet() with default boolean value
        System.out.println("Using subSet without boolean value: " + numbers.subSet(4, 6));

        // Using subSet() with specified boolean value
        System.out.println("Using subSet with boolean value: " + numbers.subSet(4, false, 6, true));
    }
}

Output

TreeSet: [2, 4, 5, 6]
Using subSet without boolean value: [4, 5]
Using subSet with boolean value: [5, 6]

Set Operations

The methods of the TreeSet class can also be used to perform various set operations.

Union of Sets

To perform the union between two sets, we use the addAll() method. For example,

import java.util.TreeSet;;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> evenNumbers = new TreeSet<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("TreeSet1: " + evenNumbers);

        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        System.out.println("TreeSet2: " + numbers);

        // Union of two sets
        numbers.addAll(evenNumbers);
        System.out.println("Union is: " + numbers);

    }
}

Output

TreeSet1: [2, 4]
TreeSet2: [1, 2, 3]
Union is: [1, 2, 3, 4]

Intersection of Sets

To perform the intersection between two sets, we use the retainAll() method. For example,

import java.util.TreeSet;;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> evenNumbers = new TreeSet<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("TreeSet1: " + evenNumbers);

        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        System.out.println("TreeSet2: " + numbers);

        // Intersection of two sets
        numbers.retainAll(evenNumbers);
        System.out.println("Intersection is: " + numbers);
    }
}

Output

TreeSet1: [2, 4]
TreeSet2: [1, 2, 3]
Intersection is: [2]

Difference of Sets

To calculate the difference between the two sets, we can use the removeAll() method. For example,

import java.util.TreeSet;;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> evenNumbers = new TreeSet<>();
        evenNumbers.add(2);
        evenNumbers.add(4);
        System.out.println("TreeSet1: " + evenNumbers);

        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        System.out.println("TreeSet2: " + numbers);

        // Difference between two sets
        numbers.removeAll(evenNumbers);
        System.out.println("Difference is: " + numbers);
    }
}

Output

TreeSet1: [2, 4]
TreeSet2: [1, 2, 3, 4]
Difference is: [1, 3]

Subset of a Set

To check if a set is a subset of another set or not, we use the containsAll() method. For example,

import java.util.TreeSet;

class Main {
    public static void main(String[] args) {
        TreeSet<Integer> numbers = new TreeSet<>();
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        numbers.add(4);
        System.out.println("TreeSet1: " + numbers);

        TreeSet<Integer> primeNumbers = new TreeSet<>();
        primeNumbers.add(2);
        primeNumbers.add(3);
        System.out.println("TreeSet2: " + primeNumbers);

        // Check if primeNumbers is subset of numbers
        boolean result = numbers.containsAll(primeNumbers);
        System.out.println("Is TreeSet2 subset of TreeSet1? " + result);
    }
}

Output

TreeSet1: [1, 2, 3, 4]
TreeSet2: [2, 3]
Is TreeSet2 subset of TreeSet1? True

Other Methods of TreeSet

To learn more, visit Java TreeSet (official Java documentation).

TreeSet Vs. HashSet

Both the TreeSet as well as the HashSet implements the Set interface. However, there exist some differences between them.

• Unlike HashSet, elements in TreeSet are stored in some order. It is because TreeSet implements the SortedSet interface as well.
• TreeSet provides some methods for easy navigation. For example, first(), last(), headSet(), tailSet(), etc. It is because TreeSet also implements the NavigableSet interface.
• HashSet is faster than the TreeSet for basic operations like add, remove, contains and size.

TreeSet Comparator

In all the examples above, tree set elements are sorted naturally. However, we can also customize the ordering of elements.

For this, we need to create our own comparator class based on which elements in a tree set are sorted. For example,

import java.util.TreeSet;
import java.util.Comparator;

class Main {
    public static void main(String[] args) {

        // Creating a tree set with a customized comparator
        TreeSet<String> animals = new TreeSet<>(new CustomComparator());

        animals.add("Dog");
        animals.add("Zebra");
        animals.add("Cat");
        animals.add("Horse");
        System.out.println("TreeSet: " + animals);
    }

    // Creating a comparator class
    public static class CustomComparator implements Comparator<String> {

        @Override
        public int compare(String animal1, String animal2) {
            int value =  animal1.compareTo(animal2);

            // elements are sorted in reverse order
            if (value > 0) {
                return -1;
            }
            else if (value < 0) {
                return 1;
            }
            else {
                return 0;
            }
        }
    }
}

Output

TreeSet: [Zebra, Horse, Dog, Cat]

In the above example, we have created a tree set passing CustomComparator class as an argument.

The CustomComparator class implements the Comparator interface.

We then override the compare() method. The method will now sort elements in reverse order.

To learn more, visit Java Comparator (official Java documentation).