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1. Introduction

TreeMap is a Map implementation that stores entries in sorted order of keys. If you need key-value storage along with automatic sorting by key, TreeMap is a strong choice.

2. Key Features

Stores key-value pairs
Keys are sorted automatically
Duplicate keys are not allowed
Values can be duplicated
Usually does not allow null keys
Is not synchronized

Example:

import java.util.Map;
import java.util.TreeMap;

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

        map.put(3, "Three");
        map.put(1, "One");
        map.put(2, "Two");

        System.out.println(map);
    }
}

Output:

{1=One, 2=Two, 3=Three}

3. How TreeMap Works

TreeMap uses a Red-Black Tree internally. That is why keys stay sorted automatically.

Because of this structure:

Insertions are O(log n)
Retrieval is O(log n)
Deletion is O(log n)

4. Natural Ordering of Keys

By default, keys are sorted using natural ordering.

Examples:

Integer keys are sorted numerically
String keys are sorted alphabetically

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

marks.put("Math", 95);
marks.put("English", 88);
marks.put("Science", 91);

System.out.println(marks);

5. Common Methods

5.1 `put()`, `get()`, `remove()`

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

map.put(101, "Navin");
map.put(102, "Kiran");

System.out.println(map.get(101));

map.remove(102);

5.2 `firstKey()` and `lastKey()`

System.out.println(map.firstKey());
System.out.println(map.lastKey());

5.3 `higherKey()`, `lowerKey()`, `ceilingKey()`, `floorKey()`

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

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

6. Iterating Over a TreeMap

for (Map.Entry<Integer, String> entry : data.entrySet()) {
    System.out.println(entry.getKey() + " -> " + entry.getValue());
}

Since keys are sorted, iteration also happens in sorted order.

7. Custom Sorting with Comparator

You can provide a custom comparator while creating the TreeMap.

import java.util.Collections;
import java.util.TreeMap;

public class Demo {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>(Collections.reverseOrder());

        map.put(1, "One");
        map.put(2, "Two");
        map.put(3, "Three");

        System.out.println(map);
    }
}

Output:

{3=Three, 2=Two, 1=One}

8. `null` Keys and Values

TreeMap generally does not allow null keys when natural ordering is used.

TreeMap<Integer, String> map = new TreeMap<>();
map.put(null, "Value"); // exception

However, null values are allowed.

map.put(1, null);

9. TreeMap vs HashMap

Feature	HashMap	TreeMap
Order	No guaranteed order	Sorted by keys
Speed	Faster on average	Slower
Internal structure	Hash table	Red-Black Tree
`null` key	Allowed	Usually not allowed

Use TreeMap when sorted keys matter.

10. Practical Example

import java.util.Map;
import java.util.TreeMap;

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

        ranks.put(3, "Riya");
        ranks.put(1, "Navin");
        ranks.put(2, "Kiran");

        for (Map.Entry<Integer, String> entry : ranks.entrySet()) {
            System.out.println(entry.getKey() + " -> " + entry.getValue());
        }
    }
}

11. Summary

TreeMap stores key-value pairs in sorted order of keys.
It uses a Red-Black Tree internally.
Operations usually take O(log n) time.
It is useful when you need sorted data or navigation methods on keys.
For custom sorting, provide a Comparator.

Written By: Shiva Srivastava

TreeMap

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