Difference between HashMap and HashTable

What is HashTable?

HashTable is a legacy class in Java that also implements the Map interface. It is similar to HashMap but is synchronized, making it thread-safe. However, this synchronization comes at the cost of performance. It does not allow null values for keys or values.

Syntax:

import java.util.Hashtable;

// Declaration
Hashtable<KeyType, ValueType> table = new Hashtable<>();

// Insertion
table.put(key, value);

// Retrieval
ValueType retrievedValue = table.get(key);

Example:

• Java

Java

import java.util.Hashtable;

public class HashTableExample {
    public static void main(String[] args) {
        // Creating a Hashtable
        Hashtable<String, Integer> ages = new Hashtable<>();

        // Adding key-value pairs
        ages.put("Science", 85);
        ages.put("Math", 90);
        ages.put("IT", 100);

        // Retrieving values
        System.out.println("Marks in IT: " + ages.get("IT"));
    }
}

You can also try this code with Online Java Compiler

Run Code

Output:

Difference Between HashMap and HashTable

HashMap and HashTable are both implementations of the Map interface in Java, designed to store key-value pairs, but they exhibit notable differences. One fundamental distinction is in their synchronization behavior. HashMap is not synchronized by default, making it suitable for scenarios where thread safety is not a primary concern, allowing for better performance. 

In contrast, HashTable is synchronized, ensuring that all its methods are thread-safe. This synchronization in HashTable is achieved through the use of synchronized methods, but it comes at the cost of increased overhead and potentially reduced performance, especially in scenarios with a high level of concurrent access.

HashMap vs HashTable

Why HashTable Doesn’t Allow null and HashMap Do?

The decision of whether to allow null values in data structures like HashTable and HashMap is influenced by design considerations and the intended use cases of these classes.

In the case of HashTable:

• Legacy Design: HashTable is a legacy class in Java that predates the Java Collections Framework. When it was designed, the Java language did not enforce strict null safety, and the use of null values was more flexible. However, the designers of HashTable chose to restrict the use of null values, likely for consistency and to avoid potential issues associated with null references.
• NullPointerException Concerns: Allowing null values in HashTable could lead to NullPointerExceptions during key or value retrieval or manipulation. By disallowing nulls, HashTable enforces a stricter contract, requiring non-null keys and values.

On the other hand, in the case of HashMap:

• Flexibility and Versatility: HashMap is part of the Java Collections Framework, introduced later with a more flexible and versatile design. The framework embraced the use of null values, providing developers with more freedom in representing data where the absence of a value is a valid state.
• Alignment with Java Collections Framework: HashMap follows the conventions of the Java Collections Framework, which allows null values in various contexts. This alignment ensures consistency and familiarity for developers working with collections in Java.

Frequently Asked Questions

What is the difference between HashMap and Hashtable and HashSet?

HashMap and Hashtable store key-value pairs, with HashMap allows nulls and are non-synchronized, while Hashtable is synchronized and doesn’t allow nulls. HashSet only stores unique elements without key-value pairs, internally using a HashMap.

Why Hashtable is faster than HashMap?

Actually, HashMap is generally faster than Hashtable because HashMap is unsynchronized, allowing multiple threads to access it concurrently. Hashtable is synchronized, which introduces overhead, making it slower in multi-threaded environments.

What is the difference between Hashtable and ConcurrentHashMap?

Hashtable is fully synchronized, locking the entire table on updates, while ConcurrentHashMap allows concurrent read and write operations by locking only portions of the map, making it more efficient in multi-threaded environments.

Conclusion

In this blog, we looked in detail about the difference between HashMap and HashTable. This blog also gave a general overview about which one to use between HashMap and HashTable. We also saw an example which showed how similar HashMap and HashTable are in implementation.

Recommended Readings:

• hash function in data structure
• entryset java
• Internal Working of HashMap

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