🗺️ Java Map Hierarchy – HashMap, TreeMap, LinkedHashMap Explained

✨ Introduction

In Java, the Map interface is part of the Java Collections Framework, but unlike List, Set, and Queue, it doesn’t extend the Collection interface.

The Map stores key-value pairs, where each key is unique, and each value is associated with a key. It’s one of the most commonly used structures in real-world projects and interviews.

🔧 Map Interface Hierarchy

Here’s a simple overview of the core Map implementations:

Map
 ├── HashMap
 ├── LinkedHashMap
 ├── TreeMap
 └── Hashtable

Each implementation offers different performance, ordering, and null-handling characteristics.

🧱 Key Implementations of Map

1️⃣ HashMap

Unordered — no guarantee on the iteration order
Allows null keys and multiple null values
Non-synchronized
Backed by a hash table

💡 When to Use:

When you need fast access, and ordering doesn’t matter.

Map<String, String> map = new HashMap<>();
map.put("A", "Apple");
map.put("B", "Banana");

⚙️ Big O:

Operation	Time Complexity
put/get/remove	O(1) average, O(n) worst
containsKey	O(1) average

2️⃣ LinkedHashMap

Maintains insertion order
Slightly slower than HashMap
Still allows null keys and values

💡 When to Use:

When you want fast lookups and predictable iteration order.

Map<String, String> map = new LinkedHashMap<>();
map.put("B", "Banana");
map.put("A", "Apple");

3️⃣ TreeMap

Sorted based on the key (natural order or comparator)
No null keys allowed (but null values are allowed)
Backed by a Red-Black tree

💡 When to Use:

When you need a sorted map or range queries.

Map<Integer, String> map = new TreeMap<>();
map.put(3, "Three");
map.put(1, "One");

⚙️ Big O:

Operation	Time Complexity
put/get/remove	O(log n)

4️⃣ Hashtable

Legacy class (before Java Collections Framework)
Thread-safe, but slower
No null keys or values allowed

⚠️ Recommendation:

Avoid in new code; use ConcurrentHashMap if you need thread safety.

🔄 Map vs Collection

Feature	Map	Collection
Key-Value Pair	✅ Yes	❌ No
Duplicates	❌ Keys must be unique	✅ Duplicates allowed (List)
Extends Collection?	❌ No	✅ Yes (List, Set, etc.)

🔐 Synchronized Variants

For thread-safe operations:

  Map<String, String> syncMap = Collections.synchronizedMap(new HashMap<>());

Or use ConcurrentHashMap for better performance in multi-threaded environments.

✅ Summary

Map Type	Ordering	Null Keys	Thread Safe	Sorted
`HashMap`	No	Yes	No	No
`LinkedHashMap`	Insertion order	Yes	No	No
`TreeMap`	Sorted order	No	No	Yes
`Hashtable`	No	No	Yes	No

📌 Final Thoughts

Understanding the Map hierarchy is crucial for writing efficient and readable Java code. Each implementation serves a unique use case:

🔍 Use HashMap for general-purpose lookups
🔄 Use LinkedHashMap when order matters
🔢 Use TreeMap when sorted keys are required
🧵 Use ConcurrentHashMap for thread-safe performance

Java Map Hierarchy

Table of contents