MediumExperience level: 3-5 years · 8-15 years1 min read

What is the difference between HashMap and Hashtable?

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Concept Made Simple

Coffee Chat Question

“What is the difference between HashMap and Hashtable?”

Remember It Faster

Mind Map Answer

Same idea — key/value storage — but built for different eras.

HashMap
Not synchronized, fast, allows 1 null key
Hashtable
Synchronized, slower, no null keys/values
Modern pick
ConcurrentHashMap for thread safety

Rule of thumb: use HashMap by default. If you need concurrency, reach for ConcurrentHashMap — Hashtable is legacy.

Learn by Doing

Hands-on Keyboard

java
Map<String, String> map = new HashMap<>();
map.put(null, "ok"); // allowed

Map<String, String> table = new Hashtable<>();
// table.put(null, "x"); // throws NullPointerException

Think Beyond the Expected

What If?

Why is Hashtable considered slow?

Every method is synchronized, so only one thread touches the table at a time. ConcurrentHashMap locks only a segment/bucket, allowing concurrent reads and writes.

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Collections (Core Java)
Interview question: "What is the difference between HashMap and Hashtable?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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