Difference Between HashMap and Hashtable in Java

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When it comes to data storage and retrieval in Java, two commonly used classes are HashMap and Hashtable. Both serve as implementations of the Map interface, facilitating the storage of key-value pairs. Despite their similar functionality, these classes have distinct differences that can significantly impact their usage in different scenarios. In this blog post, we will delve into the contrasts between HashMap and Hashtable, accompanied by code samples to illustrate their usage.

1.  Underlying Synchronization Mechanism:

One of the most notable distinctions between HashMap and Hashtable lies in their synchronization mechanism. Hashtable is synchronized, which means it is thread-safe and can be accessed by multiple threads concurrently without causing data corruption. In contrast, HashMap is not synchronized by default. This makes HashMap more efficient in single-threaded scenarios but requires manual synchronization when used in multithreaded environments.

Code Sample:

// Creating a synchronized Hashtable

Hashtable<String, Integer> hashtable = new Hashtable<>();

// Creating a non-synchronized HashMap

HashMap<String, Integer> hashMap = new HashMap<>();

2. Null Key and Value:

Hashtable does not allow null keys or values. If you attempt to insert a null key or value, it will throw a `NullPointerException`. On the other hand, HashMap permits a single null key and any number of null values, as it handles nulls by design.

Code Sample:

// This will throw a NullPointerException in Hashtable

// hashtable.put(null, 1);

// This is allowed in HashMap

hashMap.put(null, 1);

hashMap.put("key", null);

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3. Performance:

HashMap generally offers better performance than Hashtable due to its lack of synchronization. In single-threaded scenarios, HashMap's non-synchronized nature results in faster operations. However, in multithreaded environments, where synchronization is crucial to prevent data corruption, Hashtable might perform better due to its built-in synchronization.

Code Sample:

// Measure time taken to insert elements in HashMap

long startTime = System.nanoTime();

for (int i = 0; i < 1000000; i++) {

    hashMap.put("key" + i, i);

}

long endTime = System.nanoTime();

System.out.println("Time taken by HashMap: " + (endTime - startTime) + " ns");

// Measure time taken to insert elements in Hashtable

startTime = System.nanoTime();

for (int i = 0; i < 1000000; i++) {

    hashtable.put("key" + i, i);

}

endTime = System.nanoTime();

System.out.println("Time taken by Hashtable: " + (endTime - startTime) + " ns");

4. Iteration Order:

HashMap does not guarantee any specific order of iteration over its elements. The order can vary based on the internal hash codes of keys. Hashtable, however, maintains the order of insertion during iteration.

Code Sample:

// Iterate over HashMap's key-value pairs (order may vary)

for (Map.Entry<String, Integer> entry : hashMap.entrySet()) {

    System.out.println(entry.getKey() + ": " + entry.getValue());

}

// Iterate over Hashtable's key-value pairs (order of insertion is maintained)

for (Map.Entry<String, Integer> entry : hashtable.entrySet()) {

    System.out.println(entry.getKey() + ": " + entry.getValue());

}

In conclusion, both HashMap and Hashtable are used for storing key-value pairs in Java, but they exhibit crucial differences in terms of synchronization, handling of nulls, performance, and iteration order. Choosing between them depends on the specific requirements of your application. If thread-safety is paramount, Hashtable might be the preferred choice, while HashMap offers better performance in non-multithreaded scenarios. Understanding these distinctions will empower you to make informed decisions when working with these data structures in your Java applications.

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