Java Hashtable

The Java Hashtable is a legacy class that implements the Map interface. It provides a way to store and manage key-value pairs similar to a HashMap, but Hashtable has some distinct characteristics and differences, including its synchronization and null value handling. Here are key points about Hashtable:

1. Synchronization:

• Unlike HashMap, which is not synchronized by default, Hashtable is synchronized. This means that it is thread-safe, and multiple threads can safely access and modify a Hashtable concurrently without the need for external synchronization.

1. Null Keys and Values:

• Hashtable does not allow null keys or values. If you attempt to insert a null key or value, it will throw a NullPointerException.

1. Underlying Data Structure:

• Hashtable uses a hash table data structure to store key-value pairs, similar to HashMap. This allows for efficient key-based retrieval.

1. Order:

• The order of key-value pairs in a Hashtable is not guaranteed. It does not maintain the order in which elements were inserted or accessed.

1. Legacy:

• Hashtable is considered a legacy class and is not commonly used in modern Java development. Instead, HashMap is preferred for its better performance in most use cases. However, if you need thread safety and are working with older code or legacy systems, Hashtable may still be encountered.

1. Common Methods:

• put(K key, V value): Associates the specified key with the specified value in the map.
• get(Object key): Retrieves the value associated with the given key.
• remove(Object key): Removes the key-value pair associated with the given key.
• containsKey(Object key): Checks if the map contains the specified key.
• keys(): Returns an enumeration of the keys in the map.
• elements(): Returns an enumeration of the values in the map.

Here’s an example of using Hashtable:

import java.util.Hashtable;
import java.util.Enumeration;

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

        // Adding key-value pairs
        ages.put("Alice", 25);
        ages.put("Bob", 30);
        ages.put("Charlie", 35);

        // Retrieving a value
        int aliceAge = ages.get("Alice"); // Retrieves the value associated with the key "Alice" (25)

        // Iterating through keys using Enumeration
        Enumeration<String> keys = ages.keys();
        while (keys.hasMoreElements()) {
            String key = keys.nextElement();
            int value = ages.get(key);
            System.out.println(key + " is " + value + " years old.");
        }
    }
}

It’s worth noting that while Hashtable provides thread safety, in modern Java, ConcurrentHashMap is often preferred for concurrent use cases because it offers better performance and more fine-grained control over locking.