Difference between HashMap and Hashtable

Both HashMap and Hashtable are used to store and retrieve data in key-value pairs in Java, but there are some differences between them:

  1. Synchronization: Hashtable is synchronized, which means it is thread-safe and can be accessed by multiple threads simultaneously without causing any errors. HashMap, on the other hand, is not synchronized by default, but you can make it synchronized using the Collections.synchronizedMap() method.
  2. Null keys and values: Hashtable does not allow null keys or values. If you try to insert a null key or value, it will throw a NullPointerException. HashMap, on the other hand, allows null keys and values.
  3. Iteration: Hashtable provides an Enumeration interface to iterate over its elements, whereas HashMap provides an Iterator interface to iterate over its elements. The Iterator interface is more powerful than the Enumeration interface.
  4. Performance: HashMap is generally faster than Hashtable because it is not synchronized by default. However, if you need a thread-safe map, Hashtable is the way to go.
  5. Legacy: Hashtable is a legacy class that has been around since the early days of Java, whereas HashMap is a newer class that was introduced in Java 1.2. HashMap is generally preferred over Hashtable because it offers more flexibility and better performance.

In summary, if you need a thread-safe map and don’t mind the overhead of synchronization, use Hashtable. Otherwise, use HashMap.