HashMap vs Hashtable in Java

By: Rajesh P.S.

HashMap and Hashtable are both implementations of the Map interface in Java, which provide key-value pair storage and retrieval capabilities. While they share similarities in functionality, there are some significant differences between HashMap and Hashtable.

Thread-safety: Hashtable is synchronized, meaning it is designed to be thread-safe. It ensures that multiple threads can access and modify the Hashtable concurrently without encountering data corruption issues. On the other hand, HashMap is not synchronized by default, which makes it not inherently thread-safe. However, you can achieve thread-safety for HashMap by using synchronized blocks or by utilizing the concurrent ConcurrentHashMap class.
Null values and keys: Hashtable does not allow null keys or values. If you attempt to store a null key or value in a Hashtable, it will throw a NullPointerException. In contrast, HashMap permits null values and a single null key. You can store a null value and retrieve it from a HashMap without any issues.
Iteration order: HashMap does not guarantee any specific iteration order, as it is not ordered. The order of elements in a HashMap may vary over time, depending on factors such as resizing or rehashing. Hashtable, on the other hand, guarantees an iteration order that is the same as the order in which elements were inserted.
Performance: HashMap generally provides better performance compared to Hashtable, especially in scenarios where thread-safety is not a requirement. The lack of synchronization in HashMap allows for faster operations, making it more efficient for single-threaded use cases.

Following is an example to illustrate the usage of HashMap and Hashtable:

import java.util.HashMap; import java.util.Hashtable; import java.util.Map; public class MapExample { public static void main(String[] args) { // HashMap example Map<Integer, String> hashMap = new HashMap<>(); hashMap.put(1, "Apple"); hashMap.put(2, "Banana"); hashMap.put(3, "Orange"); hashMap.put(4, null); // Null value is allowed System.out.println("HashMap: " + hashMap); // Hashtable example Hashtable<Integer, String> hashtable = new Hashtable<>(); hashtable.put(1, "Red"); hashtable.put(2, "Green"); hashtable.put(3, "Blue"); // Attempting to add null key or value will throw NullPointerException // hashtable.put(4, null); System.out.println("Hashtable: " + hashtable); } }

In this example, we create a HashMap and a Hashtable to store key-value pairs. We demonstrate that HashMap allows null values (in this case, the value for key 4), while Hashtable does not permit null values. We also observe that the iteration order of HashMap may differ from the order of insertion, whereas Hashtable maintains the insertion order.

Conclusion

Consider the specific requirements of your application, such as thread-safety and null values, when choosing between HashMap and Hashtable. If you require thread-safety, Hashtable may be more appropriate, but if thread-safety is not a concern, HashMap often offers better performance.

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