Explain the difference between HashMap, HashTable, and ConcurrentHashMap.

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🔹 1. HashMap

• Introduced in: Java 1.2

• Synchronization: Not synchronized → Not thread-safe. If multiple threads access it simultaneously, external synchronization is required.

• Null keys/values: Allows 1 null key and multiple null values.

• Performance: Faster in single-threaded applications because there is no synchronization overhead.

• Use case: Best for non-threaded environments or when only one thread accesses the map at a time.

🔹 2. Hashtable

• Introduced in: Java 1.0 (legacy class).

• Synchronization: Synchronized at the method level → Thread-safe, but all operations are blocked at method calls, leading to performance bottlenecks.

• Null keys/values: Does not allow null key or null values.

• Performance: Slower than HashMap due to synchronized methods.

• Use case: Rarely used now; mostly replaced by ConcurrentHashMap in multithreaded environments.

🔹 3. ConcurrentHashMap

• Introduced in: Java 1.5

• Synchronization: Thread-safe but with better concurrency. It uses bucket-level (segment-level) locking or CAS (in newer versions), so multiple threads can read and write simultaneously without blocking the entire map.

• Null keys/values: Does not allow null key or null values (to avoid ambiguity in concurrent operations).

• Performance: Much faster than Hashtable in multi-threaded scenarios since it reduces contention.

• Use case: Ideal for highly concurrent applications where multiple threads update/read maps frequently.

✅ Quick Comparison Table

Feature	HashMap	Hashtable	ConcurrentHashMap
Thread-safe	❌ No	✅ Yes	✅ Yes (efficient)
Synchronization	None	Whole map (methods)	Partial (buckets/segments)
Null key	1 allowed	Not allowed	Not allowed
Null values	Multiple allowed	Not allowed	Not allowed
Performance	Fast (single-thread)	Slow (global lock)	Fast (multi-threaded)
Introduced in	Java 1.2	Java 1.0	Java 1.5

👉 In summary:

• Use HashMap for single-threaded applications.

• Use ConcurrentHashMap for multi-threaded applications.

• Avoid Hashtable, since it’s legacy and less efficient.

Read more :

What is the difference between abstraction and encapsulation?

How does garbage collection work in Java?

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