Following are the differences between HashMap and Hashtable class in java:

HashMap class

• HashMap class is not synchronized in java that means multiple threads can access it simultaneously.
• HashMap class extends AbstractMap class.
• HashMap class allows one null key and multiple null values.

Hashtable class

• Hashtable class is synchronized in java that means multiple threads cannot access it simultaneously.
• Hashtable class extends Dictionary class.
• Hashtable class does not allow any null key and value.