Comparing Hashtable and Synchronized Map in Java
In Java, both Hashtable and Synchronized Map are thread-safe implementations of the Map interface. However, they differ in their internal implementation, features, and behavior. This table highlights the key differences between Hashtable and Synchronized Map:
| Sr. No. | Key | Hashtable | Synchronized Map |
|---|---|---|---|
| 1 | Thread-Safety | Hashtable is thread-safe by design. All methods are synchronized. | Synchronized Map is a wrapper around a standard Map implementation (e.g., HashMap) that provides thread-safety by synchronizing the entire map. |
| 2 | Null Keys and Values | Hashtable does not allow null keys or null values. | Synchronized Map allows one null key and any number of null values, depending on the underlying Map implementation. |
| 3 | Performance | Hashtable may have lower performance compared to Synchronized Map due to the overhead of acquiring and releasing locks on every method call. | Synchronized Map can potentially perform better than Hashtable in highly concurrent scenarios due to the use of a single lock for the entire map. |
| 4 | Concurrency Level | Hashtable has a single lock for the entire table, allowing only one thread to access it at a time. | Synchronized Map also has a single lock for the entire map, but it can be wrapped around other concurrent Map implementations like ConcurrentHashMap for better concurrency levels. |
| 5 | Iterators | Iterators on Hashtable are fail-safe, meaning they will not throw ConcurrentModificationException if the Hashtable is modified during iteration. | Iterators on Synchronized Map are fail-fast, meaning they will throw ConcurrentModificationException if the underlying Map is modified during iteration. |
| 6 | Legacy | Hashtable is a legacy class that existed before Java Collections Framework was introduced. | Synchronized Map is part of the Java Collections Framework and is typically implemented using the Collections.synchronizedMap() method. |
| 7 | Ordering | Hashtable maintains an unpredictable order of elements. | Synchronized Map maintains the same order of elements as the underlying Map implementation (e.g., insertion order for HashMap). |
| 8 | Compatibility | Hashtable is compatible with older Java versions but has been replaced by more modern concurrent Map implementations like ConcurrentHashMap. | Synchronized Map is compatible with all Java versions and can be used with various Map implementations. |
| 9 | Memory Overhead | Hashtable has a higher memory overhead due to its thread-safe implementation and additional overhead for synchronization. | Synchronized Map has a lower memory overhead compared to Hashtable, as it only adds a thin synchronization layer on top of the underlying Map implementation. |
Example: Synchronized Map
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
public class SynchronizedMapExample {
public static void main(String[] args) {
Map<String, Integer> map = Collections.synchronizedMap(new HashMap<>());
// Add elements to the map
map.put("Apple", 1);
map.put("Banana", 2);
map.put("Orange", 3);
// Iterate over the map
for (Map.Entry<String, Integer> entry : map.entrySet()) {
System.out.println(entry.getKey() + ": " + entry.getValue());
}
}
}In this example, we create a Synchronized Map by wrapping a HashMap using the Collections.synchronizedMap() method. We then add elements to the map and iterate over its entries in a thread-safe manner.
While Hashtable provides thread-safety out of the box, Synchronized Map offers more flexibility by allowing you to choose the underlying Map implementation and potentially achieve better performance in highly concurrent scenarios. However, Hashtable may be preferred in legacy code or scenarios where null keys or values are not allowed.
Ultimately, the choice between Hashtable and Synchronized Map depends on your specific requirements, such as performance, concurrency levels, null handling, and compatibility with existing code.
