What is the difference between Java HashMap and Hashtable?

What is the difference between 1.HashMap and Hashtable?
HashMap is not thread-safe and its operation method is not synchronized
HashMap allow key to be empty
　　　　
Hashtable is thread-safe and its operation method has been synchronized modified
Hashtable does not allow key to be empty

What is the difference between 2.ArrayList and linkedlist and vectors?
ArrayList internal implementation is an object array, non-thread safe
LinkedList internal implementation is a linked list structure, non-thread-safe
Vector internal implementation is an object array, thread-safe

What is the difference between Java HashMap and Hashtable?