Let’s first take a look at key points about HashTable and HashMap:

Synchronization:

• HashTable: Synchronized, which means it’s thread-safe and can be safely used in multi-threaded applications without explicit synchronization.
• HashMap: Not synchronized, making it faster but not thread-safe by default.

Null Keys and Values:

• HashTable: Doesn’t allow any null key or value.
• HashMap: Allows one null key and multiple null values.

Performance:

• HashTable: Generally slower due to synchronization.
• HashMap: Typically faster since it doesn’t have the overhead of synchronization.

Inheritance:

• HashTable: Directly extends the Dictionary class.
• HashMap: Extends the AbstractMap class and implements the Map interface.

Iterators:

• HashTable: Uses Enumeration to retrieve keys and values.
• HashMap: Uses Iterator for the same purpose. The iterator in HashMap is fail-fast.

Legacy:

• HashTable: It’s a legacy class and is considered obsolete for new code. Part of Java since its first version.
• HashMap: Introduced later in Java 2. It’s part of the Java Collections Framework (JCF) and is generally recommended over HashTable for new implementations.

Load Factor:

• HashTable: The default load factor is 0.75, which is a measure of how full the hash table is allowed to get before its capacity is automatically increased.
• HashMap: Allows specifying the load factor during initialization.

Order:

• HashTable: Doesn’t guarantee that the order of the map will remain constant over time.
• HashMap: Similarly, doesn’t guarantee order, but related classes like LinkedHashMap do.

Choosing between HashTable and HashMap largely depends on the specific needs of the application, especially regarding thread safety and performance requirements. For modern applications, HashMap (or its thread-safe counterpart, ConcurrentHashMap) is generally preferred over HashTable.

Difference Between HashTable and HashMap in Java

Criteria	HashTable	HashMap
Synchronization	Synchronized	Not synchronized
Null Keys and Values	Doesn’t allow any null key or null value	Allows one null key and multiple null values
Performance	Generally slower due to synchronization	Faster because it is not synchronized
Iterating	Enumerator for iteration (though it also has Iterator)	Iterator for iteration
Fail-fast	Enumerator is not fail-fast	Iterator is fail-fast
Legacy	Yes, it’s a legacy class	No
Use-case	When thread safety is a priority, but consider using ConcurrentHashMap for a more modern approach	When thread safety is not required, or it’s controlled externally

Example: Difference Between HashTable and HashMap in Java

import java.util.HashMap;
import java.util.Hashtable;
import java.util.Map;

public class HashTableVsHashMapDemo {

    public static void main(String[] args) {

        // Create a HashMap and add some key-value pairs
        Map<String, String> hashMap = new HashMap<>();
        hashMap.put("three", "cherry");
        hashMap.put("one", "banana");
        hashMap.put("two", "apple");
        hashMap.put("four", "date");

        System.out.println("HashMap:");
        hashMap.forEach((key, value) -> System.out.println(key + " -> " + value));

        // Create a Hashtable and add some key-value pairs
        Map<String, String> hashTable = new Hashtable<>();
        hashTable.put("c", "elephant");
        hashTable.put("a", "grape");
        hashTable.put("b", "fox");
        hashTable.put("d", "honeydew");

        System.out.println("\nHashtable:");
        hashTable.forEach((key, value) -> System.out.println(key + " -> " + value));
    }
}

Output:

HashMap:
three -> cherry
one -> banana
two -> apple
four -> date

Hashtable:
a -> grape
b -> fox
c -> elephant
d -> honeydew

Explanation:

1. HashMap: A HashMap is an implementation of the Map interface that stores key-value pairs. It allows null keys and null values. It does not guarantee any specific order of the keys or values.

2. Hashtable: Hashtable is the older version of the HashMap and is synchronized. This means that it is thread-safe and can be used with multiple threads, but it brings a performance cost associated with synchronization. Hashtable does not allow null keys or values.

3. In the given example, both the HashMap and Hashtable store and retrieve key-value pairs. The insertion order in HashMap is not preserved like the Hashtable.

4. The decision to use HashMap or Hashtable often hinges on the need for synchronization. If thread safety is a concern, and you're working in a multithreaded environment, Hashtable might be considered. However, for single-threaded applications or situations where thread safety is managed externally, HashMap is generally more efficient and more commonly used in modern Java applications.