Hashtable vs HashMap

Both Hashtable and HashMap are data structures used to store key-value pairs, offering constant-time average complexity for basic operations like insertion, deletion, and retrieval. However, there are key differences between them.

Key Differences

Synchronization: Hashtable is synchronized, meaning it is thread-safe and can be used in a multithreaded environment without external synchronization. HashMap, on the other hand, is not synchronized and is not inherently thread-safe. If thread safety is required with HashMap, external synchronization mechanisms like Collections.synchronizedMap() or ConcurrentHashMap need to be used.
Null Keys and Values: Hashtable does not allow null keys or null values. Attempting to insert a null key or value will result in a NullPointerException. HashMap allows one null key and multiple null values.
Iteration: Hashtable provides iterators that are fail-fast, meaning they will throw a ConcurrentModificationException if the Hashtable is structurally modified during iteration (except through the iterator's own remove method). HashMap's iterators are also fail-fast.
Inheritance: Hashtable inherits from the Dictionary class (which is now obsolete), while HashMap inherits from the AbstractMap class.
Performance: Because Hashtable is synchronized, it generally has lower performance than HashMap in single-threaded environments. The overhead of synchronization can be significant.

When to Use Which

Hashtable: Use Hashtable when thread safety is a primary concern and performance is not critical. In modern Java applications, ConcurrentHashMap is often preferred over Hashtable for thread-safe map implementations due to its finer-grained locking and better performance.
HashMap: Use HashMap when thread safety is not required or when external synchronization is provided. It offers better performance in single-threaded environments due to the absence of synchronization overhead.

Summary Table

Feature	Hashtable	HashMap	ConcurrentHashMap
Synchronization	Synchronized (thread-safe)	Not synchronized (not thread-safe)	Thread-safe (finer-grained locking)
Null Keys	Not allowed	One null key allowed	Not allowed
Null Values	Not allowed	Multiple null values allowed	Not allowed
Inheritance	Dictionary	AbstractMap	AbstractMap
Performance	Generally slower due to synchronization	Generally faster in single-threaded apps	High concurrency and good performance

Example (Java)

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

public class MapComparison {

public static void main(String[] args) {
    // HashMap Example
    Map<String, String> hashMap = new HashMap<>();
    hashMap.put("key1", "value1");
    hashMap.put("key2", "value2");
    hashMap.put(null, "nullValue"); // Allowed
    hashMap.put("key3", null);      // Allowed
    System.out.println("HashMap: " + hashMap);

    // Hashtable Example
    Map<String, String> hashtable = new Hashtable<>();
    hashtable.put("key1", "value1");
    hashtable.put("key2", "value2");
    // hashtable.put(null, "nullValue"); // Throws NullPointerException
    // hashtable.put("key3", null);      // Throws NullPointerException
    System.out.println("Hashtable: " + hashtable);

    // ConcurrentHashMap Example
    ConcurrentHashMap<String, String> concurrentHashMap = new ConcurrentHashMap<>();
    concurrentHashMap.put("key1", "value1");
    concurrentHashMap.put("key2", "value2");
    // concurrentHashMap.put(null, "nullValue"); // Throws NullPointerException
    // concurrentHashMap.put("key3", null);      // Throws NullPointerException
    System.out.println("ConcurrentHashMap: " + concurrentHashMap);
}

}

Conclusion

Choose HashMap when thread safety is not a concern and you need optimal performance in a single-threaded environment. For thread-safe scenarios, prefer ConcurrentHashMap over Hashtable for its better concurrency and performance. Understand the nuances of null key/value handling to avoid unexpected NullPointerException errors.