What is difference between HashMap and HashTable and HashSet and TreeSet?

Hashtable
Hashtable is basically a data structure 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. So It is slow.

HashMap
Like Hashtable it also accepts key value pair.

• It allows null for both key and value. 

• It is unsynchronized. So come up with better performance(fast)

HashSet
It is from Set family.

• HashSet does not allow duplicate values. 

• It provides add method rather put method. 

• You also use its contains method to check whether the object is already available in HashSet. 

• HashSet can be used where you want to maintain a unique set but not the order. 

TreeSet
 It is also from Set family.

• Likely HashSet, TreeSet does not allow duplicate values.

• It provides add method rather put method. 

• You also use its contains method to check whether the object is already available in TreeSet. 

• TreeSet can be used where you want to maintain a unique set as well as the order.