Hashtable

Hashtable

Think of Hashtable as an old vault with strict guards 🔐 Everything is locked down, thread-safe by default… but a bit slow and outdated compared to modern options.

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🧠 What is Hashtable?

Hashtable is part of the Java Collections Framework

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

👉 It stores key → value pairs (like HashMap) 👉 But with built-in synchronization

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🔑 Key Characteristics

• ✔ Thread-safe (synchronized)
• ❌ No null key
• ❌ No null value
• ✔ Keys are unique
• ❌ No order guarantee
• Slower than HashMap due to synchronization

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🏗️ Internal Working

• Uses hashing (bucket array)
• Key → hashCode() → bucket index
• Handles collisions using chaining

👉 Every method is synchronized, so only one thread can access at a time

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⚙️ Important Methods

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

table.put("A", 1);
table.put("B", 2);

table.get("A");
table.remove("B");

table.containsKey("A");
table.containsValue(1);

table.size();

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🧠 Example

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

table.put("Java", 1);
table.put("Python", 2);

// table.put(null, 3); // ❌ throws NullPointerException

System.out.println(table);

👉 Output (order not guaranteed):

{Java=1, Python=2}

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⚔️ Hashtable vs HashMap

Feature	Hashtable 🧰	HashMap ⚡
Thread-safe	✔ Yes	❌ No
Performance	Slower	Faster
Null key	❌ No	✔ Yes
Null value	❌ No	✔ Yes
Legacy	✔ Yes	❌ No

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🎯 Interview Nuggets

• Hashtable is legacy class (introduced in Java 1.0)

• Replaced in most cases by:

  • HashMap (non-thread-safe)
  • ConcurrentHashMap (thread-safe & efficient)
  • Synchronization is at method level → causes bottleneck

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🧠 Memory Trick

Hashtable = Thread-safe but heavy (old-school lock system 🔐)