Java Hashtable: A Dive into Synchronized HashMaps
In the world of Java, when you need to store key-value pairs, HashMap is often your first thought. But what if you need thread-safety—what if multiple threads need to access and modify your map concurrently without corrupting its data? Enter Hashtable. While Hashtable might seem like an older, perhaps less frequently used, sibling to HashMap, it offers a distinct advantage: built-in synchronization. Let's explore Hashtable in detail, understanding its characteristics, how to use it, and when it's the right choice. What is Hashtable? java.util.Hashtable is a concrete implementation of the Map interface in Java. It stores data in key-value pairs and uses a hashing mechanism to efficiently store and retrieve objects. Key characteristics include: Synchronization: All public methods of Hashtable are synchronized. This means that only one thread can access a Hashtable instance at a time, ensuring data consistency in a multi-threaded environment. Nulls Not Allowed: Unlike HashMap, Hashtable does not allow null keys or null values. Attempting to insert a null key or value will result in a NullPointerException. Legacy Class: Hashtable is part of Java's legacy collections framework, dating back to Java 1.0. While functional, newer (and often more performant) synchronized map implementations like ConcurrentHashMap are generally preferred for new development. Initial Capacity and Load Factor: Hashtable uses an initial capacity (default 11) and a load factor (default 0.75). When the number of entries exceeds (capacity * load factor), the Hashtable automatically rehashes and increases its capacity.