what is the difference between hashmap and hashtable

HashMap and HashTable are both key-value based data structures in Java, but they differ in synchronization, performance, and flexibility.

Hashtable is synchronized, which means it allows only one thread to access it at a time — making it thread-safe but slower in performance. On the other hand, HashMap is not synchronized, so multiple threads can access it simultaneously, making it faster but not thread-safe by default.

Both HashMap and Hashtable provide similar methods such as put(), get(), and remove() to add, retrieve, and delete elements.

🧾 Comparison Table

💻 Example: HashMap vs Hashtable

import java.util.*;

public class HashMapVsHashtableExample {
public static void main(String[] args) {

    // ----- HashMap Example -----
    System.out.println("----- HashMap Example -----");
    HashMap<Integer, String> hashMap = new HashMap<>();
    hashMap.put(1, "Apple");
    hashMap.put(2, "Banana");
    hashMap.put(3, "Cherry");
    hashMap.put(null, "Mango");   // ✅ Allows one null key
    hashMap.put(4, null);         // ✅ Allows null values

    System.out.println("HashMap contents: " + hashMap);

    // Accessing elements
    System.out.println("Get value for key 2: " + hashMap.get(2));
    hashMap.remove(3);
    System.out.println("After removing key 3: " + hashMap);

    // ----- Hashtable Example -----
    System.out.println("\n----- Hashtable Example -----");
    Hashtable<Integer, String> hashtable = new Hashtable<>();
    hashtable.put(1, "Dog");
    hashtable.put(2, "Cat");
    hashtable.put(3, "Tiger");

    // ❌ Hashtable does not allow null keys or values
    try {
        hashtable.put(null, "Lion");   // Throws NullPointerException
    } catch (NullPointerException e) {
        System.out.println("Hashtable error: Null key not allowed!");
    }

    try {
        hashtable.put(4, null);        // Throws NullPointerException
    } catch (NullPointerException e) {
        System.out.println("Hashtable error: Null value not allowed!");
    }

    System.out.println("Hashtable contents: " + hashtable);

    // Accessing elements
    System.out.println("Get value for key 2: " + hashtable.get(2));
    hashtable.remove(3);
    System.out.println("After removing key 3: " + hashtable);
}

}