Explain HashMap vs Hashtable in Java
💡 Concept: HashMap vs Hashtable
HashMap and Hashtable are implementations of the Map interface in Java, with differences in synchronization and null handling.
📘 Quick Intro
HashMap is unsynchronized and allows null keys and values; Hashtable is synchronized and does not allow null keys or values.
🧠 Analogy
Think of Hashtable as a thread-safe safe with restricted inputs, while HashMap is a more flexible but unsafe locker.
🔧 Technical Explanation
- HashMap is not synchronized, so not thread-safe.
- Hashtable is synchronized and thread-safe.
- HashMap allows one null key and multiple null values.
- Hashtable does not allow any null key or value.
- HashMap is generally faster due to lack of synchronization overhead.
🎯 Use Cases
- ✅ Use HashMap in single-threaded or externally synchronized environments.
- ✅ Use Hashtable in legacy thread-safe code.
- ✅ Prefer ConcurrentHashMap for modern thread-safe requirements.
💻 Code Example: HashMap vs Hashtable
import java.util.*;
public class MapExample {
public static void main(String[] args) {
Map hashMap = new HashMap<>();
Map hashTable = new Hashtable<>();
hashMap.put(null, "Value1"); // Allowed
// hashTable.put(null, "Value2"); // Throws NullPointerException
System.out.println("HashMap: " + hashMap);
System.out.println("Hashtable: " + hashTable);
}
}
❓ Interview Q&A
Q1: Is HashMap synchronized?
A: No.
Q2: Does Hashtable allow null keys?
A: No.
Q3: Which is faster?
A: HashMap.
Q4: Is Hashtable thread-safe?
A: Yes.
Q5: Can HashMap contain null values?
A: Yes.
Q6: Which one is legacy?
A: Hashtable.
Q7: How to make HashMap synchronized?
A: Using Collections.synchronizedMap().
Q8: What exceptions if null key in Hashtable?
A: NullPointerException.
Q9: Can HashMap be used in multi-threaded environment?
A: Not without synchronization.
Q10: Which class implements Map interface?
A: Both.
📝 MCQs
Q1. Is HashMap synchronized?
- Yes
- No
- Sometimes
- Always
Q2. Does Hashtable allow null keys?
- Yes
- No
- Sometimes
- Always
Q3. Which is faster?
- Hashtable
- HashMap
- Both same
- None
Q4. Is Hashtable thread-safe?
- No
- Yes
- Sometimes
- Never
Q5. Can HashMap contain null values?
- No
- Yes
- Sometimes
- Never
Q6. Which one is legacy?
- HashMap
- Hashtable
- ConcurrentHashMap
- None
Q7. How to make HashMap synchronized?
- Using synchronized keyword
- Using Collections.synchronizedMap()
- Using ReentrantLock
- No way
Q8. What exceptions if null key in Hashtable?
- ClassCastException
- NullPointerException
- ArrayIndexOutOfBoundsException
- IllegalArgumentException
Q9. Can HashMap be used in multi-threaded environment?
- Yes
- No
- Not without synchronization
- Sometimes
Q10. Which class implements Map interface?
- HashMap only
- Hashtable only
- Both
- None
💡 Bonus Insight
Choosing between HashMap and Hashtable depends on synchronization needs and legacy code considerations.
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