What is the difference between Hashtable and HashMap in getting started with Java

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Difference between Hashtable and HashMap:

1. Hashtable is a subclass of Dictionary, and HashMap is an implementation class of Map interface;

2. Methods in Hashtable are synchronous, while methods in HashMap are asynchronous by default. That is, in multithreaded applications, Hashtables are safe to use without special operations; for HashMaps, additional synchronization mechanisms are required. But the synchronization problem of HashMap can be solved by a static method of Collections:

Map Collections.synchronizedMap(Map m)

This method returns a synchronized Map that encapsulates all the methods of the underlying HashMap, making the underlying HashMap safe even in a multithreaded environment.

3. In HashMap, null can be used as a key, and there can be only one such key; one or more keys can correspond to null. When the get() method returns a null value, it can mean that the key does not exist in the HashMap, or it can mean that the value corresponding to the key is null. Therefore, in HashMap, it is not possible to determine whether a key exists in HashMap by the get() method, but should be determined by the containsKey() method.

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