In Java , HashMap and Hashtable are both data structures used to store key-value pairs. They are similar in some aspects, but there are also some important differences. Here's a detailed comparison between them:
1. Thread safety:
·Hashtable: Hashtable is thread-safe, which means that multiple threads can access a Hashtable instance simultaneously without causing data inconsistency or other concurrency issues. It is thread-safe by using synchronized methods.
·HashMap: HashMap is not thread-safe. Simultaneous modification of HashMap by multiple threads may lead to inconsistent results. If you need a thread-safe HashMap, you can use the Collections.synchronizedMap() method to create a thread-safe HashMap wrapper.
2.Performance:
HashMap: Since no additional synchronization overhead is required, HashMap is usually better than Hashtable in performance. In a single-threaded environment, HashMap performs better.
·Hashtable: Due to the need for synchronization, the performance of Hashtable is usually worse than HashMap, especially in high-concurrency environments.
3.Null key and value:
·HashMap: HashMap allows both keys and values to be null. That means you can insert null as key or value into HashMap.
Hashtable: Hashtable does not allow keys or values to be null. If you try to insert a null key or value into the Hashtable, a NullPointerException will be thrown.
4.Inheritance relationship:
HashMap: HashMap is a replacement for the non-thread-safe version of Hashtable. It is part of the Java Collections Framework, located in the java.util package.
Hashtable: Hashtable is an earlier Java collection class, located in the java.util package. It is designed to provide a thread-safe hash table.
5. Traversal order:
HashMap: The traversal order of HashMap may be different in different JVM implementations, and order is not guaranteed.
·Hashtable: The traversal order of Hashtable is determined according to the hash value of the key, and order is not guaranteed.
6. Expansion:
·HashMap: HashMap uses load factor to control when to expand. The default load factor is 0.75. When the number of elements in the HashMap reaches 75% of the capacity, it will automatically expand.
·Hashtable: Hashtable automatically expands when the number of elements reaches 75% of the capacity, and the load factor is not allowed to be set.
In summary, HashMap is usually the preferred option to use in a single-threaded environment because it has better performance. However, in a multi-threaded environment, if thread safety is required, consider using a Hashtable, or use a more modern thread-safe collection class such as ConcurrentHashMap. At the same time, note that when choosing HashMap or Hashtable, the choice should be made according to specific needs and performance requirements.