What is the difference between Hashtable and hashmap in Java

Legacy HashMap was introduced in JDK 1.2 and extends AbstractMap class whereas Hashtable is a legacy class which initially was not part of collection framework. Later from JDK 1.2 it became part of Collection framework and implemented Map interface. Dictionary is the super class of Hashtable class. Synchronized HashMap is not synchronized whereas Hashtable is synchronized. But you can explicitly synchronize HashMap. Hence, Hashtable is thread-safe whereas HashMap is not thread-safe. Keys & Values Hashtable does not allow null keys and null values whereas HashMap allows to store one null key and any number of null values. Read full article at Difference between Hashmap and Hashtable in Java