Examples of Java’s HashMap, TreeMap, and LinkedHashMap

Examples of Java’s HashMap, TreeMap, and LinkedHashMap#

1. HashMap#

Characteristics:

  • Unordered collection (no guarantees on order).

  • Fastest performance for basic operations (get, put) with O(1) average complexity.

  • Allows null as key/value.

Example:

import java.util.HashMap;
import java.util.Map;

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

        map.put("apple", 3);
        map.put("banana", 2);
        map.put("orange", 5);

        System.out.println("Value of apple: " + map.get("apple"));

        System.out.println("Iterating HashMap:");
        for (String key : map.keySet()) {
            System.out.println("Key: " + key + ", Value: " + map.get(key));
        }
    }
}

Best Uses:

  • When you need fast lookup and insertion.

  • Order doesn’t matter.

2. TreeMap#

Characteristics:

  • Sorted according to natural ordering or a specified comparator.

  • O(log n) complexity for put, get, remove operations.

  • Cannot contain null keys (but can have null values).

Example:

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

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

        map.put("apple", 3);
        map.put("banana", 2);
        map.put("orange", 5);

        System.out.println("Value of banana: " + map.get("banana"));

        System.out.println("Iterating TreeMap (sorted):");
        for (String key : map.keySet()) {
            System.out.println("Key: " + key + ", Value: " + map.get(key));
        }
    }
}

Best Uses:

  • Maintaining sorted order (such as alphabetical order).

  • Range queries (subMap(), headMap(), tailMap()).

3. LinkedHashMap#

Characteristics:

  • Maintains insertion order.

  • Slightly slower than HashMap, with O(1) average complexity.

  • Allows null key/value.

Example:

import java.util.LinkedHashMap;
import java.util.Map;

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

        map.put("apple", 3);
        map.put("banana", 2);
        map.put("orange", 5);

        System.out.println("Value of orange: " + map.get("orange"));

        System.out.println("Iterating LinkedHashMap (insertion order):");
        for (String key : map.keySet()) {
            System.out.println("Key: " + key + ", Value: " + map.get(key));
        }
    }
}

Best Uses:

  • Caching implementations, where insertion/access order is important.

  • Preserving the order of inserted keys (e.g., configurations, ordered tasks).

Summary (Quick Reference):#

Feature

HashMap

TreeMap

LinkedHashMap

Order

No order guarantee

Sorted

Insertion order

Performance (get, put)

O(1) avg.

O(log n)

O(1) avg.

Null Keys Allowed

Yes

No

Yes

References: