HashMap vs Hashtable vs HashSet

Hashtable

Hashtable is basically a datastructure to retain values of key-value pair.

It didn’t allow null for both key and value. You will get NullPointerException if you add null value.
It is synchronized. So it comes with its cost. Only one thread can access in one time.

HashMap

Like Hashtable it also accepts key value pair.

It allows null for both key and value (It allows only one null key and multiple null values)
It is unsynchronized. So come up with better performance

HashSet

HashSet does not allow duplicate values. It provides add method rather put method. You also use its contain method to check whether the object is already available in HashSet. HashSet can be used where you want to maintain a unique list.

HashMap vs Hashtable

Both provide key-value access to data. The Hashtable is one of the original collection classes in Java. HashMap is part of the new Collections Framework, added with Java 2, v1.2.

The key difference between the two is that access to the Hashtable is synchronized on the table while access to the HashMap isn't. You can add it, but it isn't there by default.

Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't. If you change the map while iterating, you'll know.

And, a third difference is that HashMap permits null values in it, while Hashtable doesn't.