HashMap vs HashTable in Java


In Java Collections Framework, HashMap and HashTable are used to store data in key-value pairs. Although both look similar, they differ in performance, synchronization, and modern usage.

Understanding the difference between HashMap and HashTable is very important for students and developers because it is a frequently asked Java interview question.

✅ What is HashMap?

A HashMap is a part of the Java Collections Framework introduced in Java 1.2. It stores data using a hashing mechanism and allows one null key and multiple null values.

Example:

import java.util.HashMap;

HashMap<Integer, String> map = new HashMap<>();
map.put(1, "Java");
map.put(2, "Spring");
map.put(null, "Hibernate");

System.out.println(map);

Key Features:

  • Not synchronized

  • Faster performance

  • Allows one null key

  • Allows multiple null values

  • Preferred in modern applications

✅ What is HashTable?

A HashTable is a legacy class introduced in Java 1.0. It is synchronized, which makes it thread-safe but slower compared to HashMap.

Example:

import java.util.Hashtable;

Hashtable<Integer, String> table = new Hashtable<>();
table.put(1, "Java");
table.put(2, "Spring");

// table.put(null, "Hibernate"); // Null not allowed

System.out.println(table);

Key Features:

  • Synchronized (thread-safe)

  • Slower performance

  • Does not allow null key or null value

  • Legacy class

📊 Difference Between HashMap and HashTable



✅ When Should You Use Each?

  • Use HashMap for single-threaded or high-performance applications.

  • Use HashTable only when thread safety is required (though today ConcurrentHashMap is preferred).

🎯 Interview Tip

Interviewers often ask:

👉 Why is HashMap faster than HashTable?

Because HashMap is not synchronized, so it avoids locking overhead during operations.

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