Explain HashMap vs Hashtable.

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🔹 HashMap vs Hashtable in Java

Both HashMap and Hashtable store data in key-value pairs, and both use hashing for fast access. However, they differ in several key aspects.

✅ HashMap

Introduced in Java 1.2

Not synchronized → Not thread-safe

Allows one null key and multiple null values

Faster than Hashtable in single-threaded environments

Part of the Java Collections Framework

Example:

java

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HashMap<Integer, String> map = new HashMap<>();

map.put(null, "NullKey");

map.put(1, "One");

✅ Hashtable

Introduced in Java 1.0

Synchronized → Thread-safe

Does not allow null keys or values

Slower due to synchronization

Legacy class (not part of Java Collections Framework)

Example:

java

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Hashtable<Integer, String> table = new Hashtable<>();

table.put(1, "One");

// table.put(null, "NullKey"); → Throws NullPointerException

📊 Key Differences:

Feature                  HashMap              Hashtable

Thread-Safe         ❌ No                     ✅ Yes

Null Key/Value      ✅ Allowed             ❌ Not Allowed

Performance        ✅ Faster                     ❌ Slower

Introduced In        Java 1.2                      Java 1.0

Part of Collection  ✅ Yes                          ❌ No (Legacy)

📝 Summary

Use HashMap in non-threaded environments for better performance. Choose Hashtable (or better, use ConcurrentHashMap) if you need thread safety.

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Read more: 

What is the difference between List, Set, and Map?

Explain polymorphism with examples.

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