Java TreeMap

TreeMap is a Map implementation that stores its key-value entries in a self-balancing tree, keeping the entries sorted by key at all times.

In this page:

Introduction to TreeMap
Automatic Key Sorting
Adding and Removing Entries
Navigation Methods
Custom Sort Order with Comparator

Introduction to TreeMap

TreeMap is an implementation of the Map interface that stores its key-value entries in a red-black tree, keeping the entries sorted by key at all times rather than in insertion or hash order.

Example: Introduction to TreeMap

java

import java.util.TreeMap;
public class Main {
	public static void main(String[] args) {
		TreeMap<String, Integer> map = new TreeMap<>(); // red-black tree, sorted by key
		map.put("banana", 1);
		map.put("apple", 2);
		System.out.println(map);
	}
}

Automatic Key Sorting

Entries added to a TreeMap are automatically kept sorted by their key's natural ordering -- numeric keys sort numerically and String keys sort alphabetically -- regardless of the order they were inserted in.

Example: Automatic Key Sorting

java

import java.util.TreeMap;
public class Main {
	public static void main(String[] args) {
		TreeMap<Integer, String> map = new TreeMap<>();
		map.put(3, "c");
		map.put(1, "a");
		map.put(2, "b");
		System.out.println(map); // sorted by key: 1, 2, 3
	}
}

Adding and Removing Entries

TreeMap supports the same put, get, and remove operations as any other Map, but every insertion or removal also re-balances the underlying tree to keep the key order intact, and putting an existing key still overwrites its value.

Example: Adding and Removing Entries

java

import java.util.TreeMap;
public class Main {
	public static void main(String[] args) {
		TreeMap<String, Integer> map = new TreeMap<>();
		map.put("apple", 3);
		map.put("apple", 5); // overwrites
		map.remove("apple");
		System.out.println(map);
	}
}

Navigation Methods

TreeMap adds navigation methods beyond a plain Map, like firstKey, lastKey, higherKey, and floorKey, which use the guaranteed sort order to efficiently find the smallest, largest, or nearest keys relative to a given value.

Example: Navigation Methods

java

import java.util.TreeMap;
public class Main {
	public static void main(String[] args) {
		TreeMap<Integer, String> map = new TreeMap<>();
		map.put(1, "a"); map.put(5, "b"); map.put(10, "c");
		System.out.println(map.firstKey());
		System.out.println(map.lastKey());
		System.out.println(map.higherKey(5));
	}
}

Custom Sort Order with Comparator

Passing a Comparator to a TreeMap's constructor overrides the default natural key ordering, letting entries be sorted by any custom rule, such as reverse order or a computed property like key length instead of alphabetical order.

Example: Custom Sort Order with Comparator

java

import java.util.TreeMap;
import java.util.Collections;
public class Main {
	public static void main(String[] args) {
		TreeMap<Integer, String> map = new TreeMap<>(Collections.reverseOrder());
		map.put(1, "a");
		map.put(5, "b");
		System.out.println(map); // reverse sorted
	}
}

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Java TreeMap

Introduction to TreeMap

Automatic Key Sorting

Adding and Removing Entries

Navigation Methods

Custom Sort Order with Comparator

Chapter Quiz — Complete all 17 topics to unlock

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