Difference Between Hashtable and Synchronized Map in Java

This article explores the key distinctions between Java's Hashtable and SynchronizedMap. A Hashtable efficiently maps keys to values using an array of slots accessed via index calculation, providing fast data retrieval. Conversely, a SynchronizedMap synchronizes a map to ensure thread safety, preventing concurrent access issues. Crucially, SynchronizedMap does not permit null values.

Input Example

<code>[ARB, RDD, KOL, DHKA]</code>

Output Example

<code>Hashtable Insertion Order: [ARB, RDD, KOL, DHKA]
SynchronizedMap Insertion Order: [ARB, RDD, DHKA, KOL]</code>

Methodology

The comparison involves analyzing insertion order using ArrayLists and Sets to track element placement. A timed loop is used for iterative processing.

Algorithm Steps:

1. Initialize input/output streams.
2. Import necessary classes and define functions.
3. Create a public class.
4. Define functions for insertion order tracking.
5. Populate an ArrayList with input data.
6. Utilize Sets (e.g., LinkedHashSet for insertion order preservation) to manage data.
7. Print insertion order for both Hashtable and SynchronizedMap.
8. Implement a timed loop for iterative processing.
9. Output the results.

Syntax Example (Illustrative)

The following snippet demonstrates the use of TreeSet, IdentityHashMap, and other relevant classes to illustrate the concepts. Note that this is a simplified example and not a complete implementation of the algorithm described above.

TreeSet<Integer> stTree = new TreeSet<>();
stTree.add(4); stTree.add(5); stTree.add(6); stTree.add(8); stTree.add(4);

IdentityHashMap<Integer, String> ihmap = new IdentityHashMap<>();
ihmap.put(10, "ARB"); ihmap.put(20, "RDD"); ihmap.put(30, "ARBRDD");
ihmap.put(40, "KOLDHKA"); ihmap.put(50, "You");

System.out.println("IdentityHashMap size: " + ihmap.size());
System.out.println("Initial IdentityHashMap: " + ihmap);

Hashtable<Integer, String> table = new Hashtable<>();
table.put(1, "X"); table.put(2, "Y"); table.put(3, "Z"); table.put(4, "A");

for (Map.Entry<Integer, String> m : table.entrySet()) {
    Iterator<Map.Entry<Integer, String>> itr = ihmap.entrySet().iterator();
    while (itr.hasNext()) {
        IdentityHashMap.Entry<Integer, String> entry = itr.next();
        TreeMap<Integer, Integer> maptree = new TreeMap<>();
        maptree.put(2, 5); maptree.put(3, 6); maptree.put(4, 6); maptree.put(2, 3);
    }
}

Hashtable vs. SynchronizedMap Comparison

Feature

Feature	Hashtable	SynchronizedMap
Data Structure	Uses a hash table for key-value storage.	Wraps an existing map for thread safety.
Element Order	Arbitrary	Retains the original map's insertion order.
Thread Safety	Inherently synchronized (can be slow).	Synchronized using Collections.synchronizedMap()
Performance	Can be slower due to inherent synchronization.	Performance overhead due to synchronization.
Null Values	Does not allow null keys or values.	Inherits null value handling from wrapped map.

Data Structure

Uses a hash table for key-value storage.

Wraps an existing map for thread safety.

Element Order

Arbitrary

Retains the original map's insertion order.

Thread Safety

Inherently synchronized (can be slow).

Synchronized using Collections.synchronizedMap()

Performance

Can be slower due to inherent synchronization.

Performance overhead due to synchronization.

Null Values

Does not allow null keys or values.

Inherits null value handling from wrapped map.

Hashtable provides built-in thread safety but might suffer performance penalties. SynchronizedMap offers a more flexible approach, allowing you to synchronize any map implementation, but it introduces a performance overhead. Hashtable is generally considered a legacy class, and SynchronizedMap or concurrent collections (like ConcurrentHashMap) are preferred for modern Java development.

Object-Level Synchronization

Object-level synchronization, using the synchronized keyword, ensures that only one thread can access a specific object's methods at a time.

Example using Object-Level Synchronization

This example demonstrates synchronizing a HashMap using Collections.synchronizedMap().

<code>[ARB, RDD, KOL, DHKA]</code>

Conclusion

Choosing between Hashtable and SynchronizedMap depends on your specific needs. For thread safety in modern Java applications, consider using ConcurrentHashMap for better performance compared to SynchronizedMap or Hashtable. Hashtable is generally discouraged for new code.

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