What is difference between HashTable and HashMap.

• HashMap works on the principal of hashing.
• HashMap uses the hashCode() method to calculate a hash value. Hash value is calculated using the key object. This hash value is used to find the correct bucket where Entry object will be stored.
• HashMap uses the equals() method to find the correct key whose value is to be retrieved in case of get() and to find if that key already exists or not in case of put().
• Hashing collision means more than one key having the same hash value, in that case Entry objects are stored as a linked-list with in a same bucket.
• With in a bucket values are stored as Entry objects which contain both key and value.
• In Java 8 hash elements use balanced trees instead of linked lists after a certain threshold is reached while storing values. This improves the worst case performance from O(n) to O(log n).

  

     

HashTable	HashMap
Methods of Hashtable are synchronized by default	Methods are not synchronized by default
Does not permit null values (trying to add, throws NullPointerException)	Permits null values (either key or value)
Enumeration interface is used for iterating keys	Iterator is used for iterating the keys
Enumerator is not fail-fast	Iterator is fail-fast
Safe for multithreaded applications	Better for non-threaded applications
Low performance due to synchronization	High performance
Hashtable is serialized	HashMap is not serialized