Java’s TreeMap.tailMap() Method Explained

For Explanation watch

Understanding TreeMap.tailMap() in Detail

The TreeMap.tailMap() method in Java provides a powerful way to work with subsets of a TreeMap where the keys are greater than or equal to a specified key. It's an essential method for range-based operations on sorted maps, especially when the data is ordered and needs partial access.

Method Variants

Default (Inclusive by Default):

SortedMap<K, V> tailMap(K fromKey)

fromKey: The starting key for the tail view (inclusive).

Returns a view of the map with keys >= fromKey.

With Inclusivity Option:

NavigableMap<K, V> tailMap(K fromKey, boolean inclusive)

fromKey: The starting key.
inclusive: If true, includes fromKey; otherwise, excludes it.
Returns a view of the map with keys >= (or >) fromKey based on inclusive.

Core Characteristics

Dynamic View:

The returned map is not a copy but a view, meaning changes in the original TreeMap reflect in the tail map, and vice versa.

Ordering:

The elements in the tail map are ordered based on the comparator used in the original TreeMap or natural ordering if no comparator is provided.

Edge Cases:

Handles cases where:
fromKey is smaller, equal to, or larger than the smallest/largest keys.
The map is empty or contains keys not matching fromKey.

**Scenarios and Examples

1. Basic Usage**

import java.util.*;

public class BasicTailMapExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(1, "A");
        map.put(2, "B");
        map.put(3, "C");
        map.put(4, "D");

        // Tail map from key 2 (inclusive)
        SortedMap<Integer, String> tailMap = map.tailMap(2);
        System.out.println("TailMap: " + tailMap);
    }
}

Output:

TailMap: {2=B, 3=C, 4=D}

2. Using the Inclusive Option

import java.util.*;

public class TailMapInclusiveExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(10, "X");
        map.put(20, "Y");
        map.put(30, "Z");

        // Tail map excluding 20
        NavigableMap<Integer, String> tailMap = map.tailMap(20, false);
        System.out.println("TailMap (exclusive): " + tailMap);
    }
}

Output:

TailMap (exclusive): {30=Z}

3. Modifications to the Original Map

import java.util.*;

public class TailMapModificationExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(1, "One");
        map.put(2, "Two");
        map.put(3, "Three");

        SortedMap<Integer, String> tailMap = map.tailMap(2);
        System.out.println("Original TailMap: " + tailMap);

        // Modify original map
        map.put(4, "Four");
        map.remove(2);

        System.out.println("Modified Original Map: " + map);
        System.out.println("Updated TailMap: " + tailMap);
    }
}

Output:

Original TailMap: {2=Two, 3=Three}
Modified Original Map: {1=One, 3=Three, 4=Four}
Updated TailMap: {3=Three, 4=Four}

4. Handling Non-Existing Keys

If the fromKey does not exist, the map starts from the next higher key.

import java.util.*;

public class NonExistingKeyExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(5, "A");
        map.put(15, "B");
        map.put(25, "C");

        // Tail map from non-existing key 20
        SortedMap<Integer, String> tailMap = map.tailMap(20);
        System.out.println("TailMap from 20: " + tailMap);
    }
}

output :

TailMap from 20: {25=C}

5. Empty Tail Map
If fromKey is greater than the largest key, the tail map is empty.

import java.util.*;

public class EmptyTailMapExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(10, "Ten");

        // Tail map from key greater than all keys
        SortedMap<Integer, String> tailMap = map.tailMap(20);
        System.out.println("Empty TailMap: " + tailMap);
    }
}

Output:

Empty TailMap: {}

6. NullPointerException

If the TreeMap uses natural ordering and a null key is passed, a NullPointerException is thrown.

import java.util.*;

public class NullKeyExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();
        map.put(1, "A");

        try {
            map.tailMap(null);
        } catch (NullPointerException e) {
            System.out.println("Exception: " + e);
        }
    }
}

Output:

Exception: java.lang.NullPointerException

When to Use TreeMap.tailMap()

Range Queries: Retrieve all elements greater than or equal to a threshold key.

Dynamic Views: Filter and work with a live subset of the map.
Data Streaming: Efficiently stream data starting from a specific key in sorted order.

By combining it with other TreeMap methods like headMap() and subMap(), tailMap() enables flexible and precise manipulation of sorted data structures.

Comments

[Rakesh KR]

TreeMap.tailMap(K fromKey) returns a view of the portion of the map whose keys are greater than or equal to the specified fromKey. Changes to the returned map are reflected in the original map and vice versa. If a specific range is needed, the optional tailMap(K fromKey, boolean inclusive) allows finer control over inclusion of the starting key.

[Igor Venturelli]

Very good