TreeMap in Java

1. Introduction to TreeMap

TreeMap is a Java class that implements the NavigableMap interface. It stores key-value pairs sorted by keys.

Syntax

TreeMap<KeyType, ValueType> map = new TreeMap<>();

Example

import java.util.TreeMap;

public class Main {
    public static void main(String[] args) {
        TreeMap<Integer, String> students = new TreeMap<>();

        students.put(3, "Mani");
        students.put(1, "Arun");
        students.put(2, "Priya");

        System.out.println(students);
    }
}

Output:

{1=Arun, 2=Priya, 3=Mani}

2. Creating a TreeMap

Creates an empty TreeMap.

TreeMap<String, Integer> scores = new TreeMap<>();

3. Adding Elements Using put()

The put() method adds or updates a key-value pair.

TreeMap<Integer, String> map = new TreeMap<>();

map.put(2, "Java");
map.put(1, "Python");
map.put(2, "Spring Boot");

System.out.println(map); // {1=Python, 2=Spring Boot}

4. Accessing Values Using get()

The get() method returns the value associated with a key.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");

System.out.println(map.get(1)); // Java

5. Checking Keys Using containsKey()

Checks whether a key exists.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");

System.out.println(map.containsKey(1)); // true

6. Checking Values Using containsValue()

Checks whether a value exists.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");

System.out.println(map.containsValue("Java")); // true

7. Removing Elements Using remove()

Removes an entry using its key.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");
map.put(2, "Python");

map.remove(1);

System.out.println(map); // {2=Python}

8. Updating Values Using replace()

Updates the value of an existing key.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");

map.replace(1, "Spring Boot");

System.out.println(map.get(1)); // Spring Boot

9. Finding the Size Using size()

Returns the number of key-value pairs.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");
map.put(2, "Python");

System.out.println(map.size()); // 2

10. Checking Whether a TreeMap Is Empty

The isEmpty() method checks whether the map has any entries.

TreeMap<String, Integer> map = new TreeMap<>();

System.out.println(map.isEmpty()); // true

11. Removing All Entries Using clear()

Removes every key-value pair.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");

map.clear();

System.out.println(map); // {}

12. Iterating Through a TreeMap

Use entrySet() to access keys and values in sorted-key order.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(3, "C");
map.put(1, "Java");
map.put(2, "Python");

for (var entry : map.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

Output:

1 = Java
2 = Python
3 = C

13. Getting All Keys and Values

Use keySet() for keys and values() for values.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(2, "Python");
map.put(1, "Java");

System.out.println(map.keySet()); // [1, 2]
System.out.println(map.values()); // [Java, Python]

14. Finding the First and Last Entries

The firstKey() and lastKey() methods return the smallest and largest keys.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(10, "Java");
map.put(20, "Python");
map.put(30, "C++");

System.out.println(map.firstKey()); // 10
System.out.println(map.lastKey());  // 30

15. Using lowerKey(), higherKey(), floorKey() and ceilingKey()

These methods find keys relative to a specified key.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(10, "A");
map.put(20, "B");
map.put(30, "C");

System.out.println(map.lowerKey(20));   // 10
System.out.println(map.higherKey(20));  // 30
System.out.println(map.floorKey(20));   // 20
System.out.println(map.ceilingKey(25)); // 30

• lowerKey() — greatest key strictly below the given key.

• higherKey() — smallest key strictly above the given key.

• floorKey() — greatest key less than or equal to the given key.

• ceilingKey() — smallest key greater than or equal to the given key.

16. Sorting Entries in Descending Order

The descendingMap() method returns a reverse-order view of the map.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(1, "Java");
map.put(2, "Python");
map.put(3, "C++");

System.out.println(map.descendingMap());
// {3=C++, 2=Python, 1=Java}

17. Does TreeMap Allow Duplicate Keys and Values?

TreeMap does not allow duplicate keys, but different keys can have the same value.

TreeMap<Integer, String> map = new TreeMap<>();

map.put(1, "Java");
map.put(2, "Java");
map.put(1, "Python");

System.out.println(map); // {1=Python, 2=Java}

18. Does TreeMap Allow Null Keys and Values?

A TreeMap using natural key ordering does not allow null keys. It allows null values.

TreeMap<Integer, String> map = new TreeMap<>();

map.put(1, null);
map.put(2, "Java");

// map.put(null, "Python"); // Throws NullPointerException

System.out.println(map); // {1=null, 2=Java}

19. TreeMap vs HashMap vs LinkedHashMap

20. Important TreeMap Methods

21. Time Complexity of TreeMap

Conclusion

TreeMap is useful when you need to store key-value pairs with keys sorted automatically.

Key points to remember:

• Keys are unique; values can be duplicated.

• Entries are sorted by key by default.

• Natural ordering does not allow null keys, but null values are allowed.

• Adding, retrieving, and removing entries typically take O(log n).

• Use HashMap for fast average lookups without ordering, LinkedHashMap to preserve insertion order, and TreeMap for sorted keys.

Audience Engagement: Which would you choose for a Java project—HashMap, LinkedHashMap, or TreeMap—and why?