A Hashtable is a legacy class in Java that implements a Map. It stores key–value pairs and is synchronized, meaning it is thread-safe. Keys and values cannot be null.

• Stores key-value pairs.
• No null key or null value is allowed.
• Synchronized (thread-safe but slower than HashMap).
• Belongs to the java.util package.

• HashMap → Allows one null key and multiple null values, not synchronized (faster).
• Hashtable → Does not allow null keys/values, synchronized (thread-safe but slower).

• put(key, value) → Adds a key-value pair.
• get(key) → Retrieves value by key.
• remove(key) → Removes mapping for a key.
• containsKey(key) / contains(value) → Checks existence.
• keys() / elements() → Returns Enumeration of keys/values.

Use Hashtable when thread safety is required without needing external synchronization. For non-thread-safe applications, HashMap is preferred for performance.