Difference between Hashmap and Hashtable

The objective of the Hashmap and Hashtable is to store key/value pairs in a hash table. When using a Hashtable or HashMap, users have to specify an object used as a key and the value that needs to link to that key. The key is then hashed, and the subsequent hash code use as the index at which the value store within the table.

Hashmap vs. Hashtable 

• HashMap considers a non-synchronized property. However, it is not thread-safe and can’t share among many threads without proper synchronization code. On the other hand, Hashtable is a synchronized property. In addition, it is thread-safe and can share with many threads. 
• HashMap can allow one null key and multiple null values; however, Hashtable doesn’t allow any null key or value. 
• The HashMap typically favored over HashTable if thread synchronization not need.
• HashMap implementation named “LinkedHashMap” tends to maintain the insertion order, and “TreeMap” can sort the mappings based on the ascending order of keys.
• Hashtable doesn’t offer any assurance regarding any order. Also, it doesn’t maintain the mappings in any particular order.
• At the start, Hashtable was not part of the collection framework. It has been made a collection framework member after being retrofitted for the implementation of the Map interface. On the other hand, HashMap implements Map interface and is a part of the collection framework from the beginning. 
• Another variance among these classes is that the Iterator of the HashMap is a fail-fast. It throws “ConcurrentModificationException” whenever any other Thread adjusts the map mechanically by adding or taking away any element except the operator’s own remove () method. In Simple words fail-fast means: When calling iterator.next(), if any modification has been prepared between the moment the iterator shape and the moment next() call a “ConcurrentModificationException ” instantaneous thrown. The enumerator for the Hashtable is not fail-fast.

Why HashTable doesn’t allow null and do? 

For successful data storage and retrieval of objects from a HashTable, the objects used as keys must implement the hashCode technique and the equals method. As null is not an object, it can’t implement these approaches. HashMap is an innovative version of Hashtable. However, HashMap generates later.

Also read: Difference between real and virtual image

When to use HashMap and Hashtable?

As it is clarified that the main difference between HashMap & Hashtable exists in their synchronization. If there is a need for thread-safe operation. Then Hashtable will the better option as all its methods synchronize. But it’s a legacy class that should avoid because there is nothing about it. Which HashMap cannot do. For a multithreaded environment. It recommends to use ConcurrentHashMap. Which is Almost similar to Hashtable. Or even you can make the HashMap synchronized explicitly.

Furthermore, the Synchronized operation also delivers poor performance, so it should avoid unless there is an intense requirement to use it. Therefore, for a non-thread environment, HashMap should use without any doubt.