HashMap vs HashTable in Java

In this post, we will learn the difference between HashMap and HashTable in Java. This is a frequently asked question in Java interviews for beginners. Let's dive into it.

Difference between HashMap and HashTable in Java

Features	HashMap	HashTable
Synchronization	Not synchronized (not thread-safe)	Internally synchronized (thread-safe)
Null Elements	Allows one null key and any number of null values	Doesn't allow null keys and null values
Iterators	Returns fail-fast iterators	Returns fail-safe Enumerations
Class Extension	Extends AbstractMap class	Extends Dictionary class
Traversal	Returns only iterator	Returns both Iterator and Enumeration
Speed	Fast	Slow
Legacy Class	Not a legacy class	Is a legacy class
Preferred Use	Preferred in single-threaded applications; for multi-threaded, wrap using Collections.synchronizedMap()	Not preferred nowadays for multi-threaded applications; ConcurrentHashMap is a better option

Example with HashMap:

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        // Create a HashMap
        HashMap<Integer, String> hMap = new HashMap<>();

        // Add elements (key-value pairs)
        hMap.put(1, "apple");
        hMap.put(2, "banana");
        hMap.put(3, null);
        hMap.put(null, "cherry");

        // Display the HashMap
        System.out.println("HashMap:");
        for (Integer key : hMap.keySet()) {
            System.out.println(key + " => " + hMap.get(key));
        }
    }
}

Output:

HashMap:
null => cherry
1 => apple
2 => banana
3 => null

Example with HashTable:

import java.util.Hashtable;

public class HashTableExample {
    public static void main(String[] args) {
        // Create a HashTable
        Hashtable<Integer, String> hTable = new Hashtable<>();

        // Add elements (key-value pairs)
        hTable.put(1, "apple");
        hTable.put(2, "banana");
        hTable.put(3, "cherry");
        // hTable.put(null, "date"); // This will throw NullPointerException
        // hTable.put(4, null);      // This will also throw NullPointerException

        // Display the HashTable
        System.out.println("HashTable:");
        for (Integer key : hTable.keySet()) {
            System.out.println(key + " => " + hTable.get(key));
        }
    }
}

Output:

HashTable:
1 => apple
2 => banana
3 => cherry

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Related Collections Interview QA

1. map() vs flatMap() in Java
2. Collections vs Streams
3. ArrayList vs Vector
4. Iterator vs ListIterator
5. HashMap vs HashTable
6. HashSet vs HashMap
7. Array vs ArrayList
8. Fail-Fast Iterators vs Fail-Safe Iterators
9. HashMap vs ConcurrentHashMap
10. LinkedList vs ArrayDeque
11. LinkedList vs Array
12. LinkedList vs Doubly LinkedList
13. Enum vs EnumSet in Java
14. HashMap vs. TreeMap in Java
15. Synchronized Collections vs. Concurrent Collections