Understanding Java HashTable

Introduction

In Java, a HashTable is a data structure that efficiently stores key-value pairs, allowing for quick access to values based on their associated keys. This guide covers the essential features, methods, and usage of HashTable.

Key Concepts

• Key-Value Pair: A HashTable stores data in pairs, where each key is unique and maps to a specific value.
• Null Values: Unlike HashMap, HashTable does not allow null keys or null values. Any attempt to insert a null key or value will result in a NullPointerException.
• Synchronized: HashTable is synchronized, meaning it is thread-safe and can be used in multi-threaded environments without additional synchronization.
• Performance: The performance of HashTable is generally slower than HashMap because of the overhead associated with synchronization.

Key Methods

• Constructor: HashTable() - Creates an empty hash table with the default initial capacity and load factor.
• Put Method: put(K key, V value) - Adds a key-value pair to the table.
• Get Method: get(Object key) - Retrieves the value associated with the specified key.
• Remove Method: remove(Object key) - Removes the key-value pair associated with the specified key.
• Size Method: size() - Returns the number of key-value pairs in the hash table.

Example Usage

import java.util.Hashtable;

public class Example {
    public static void main(String[] args) {
        // Create a HashTable
        Hashtable<String, String> table = new Hashtable<>();

        // Add key-value pairs
        table.put("1", "Apple");
        table.put("2", "Banana");
        table.put("3", "Cherry");

        // Retrieve a value
        String value = table.get("2"); // Returns "Banana"
        System.out.println("Value for key '2': " + value);

        // Remove a key-value pair
        table.remove("1");

        // Size of the HashTable
        System.out.println("Size of HashTable: " + table.size()); // Returns 2
    }
}

Conclusion

HashTable is a valuable data structure in Java for storing key-value pairs, particularly in scenarios that require thread safety. However, for many applications, HashMap is preferred due to its better performance and flexibility with null keys and values.