Differences between HashMap and HashTable in Java.txt

Differences between HashMap and HashTable in Java
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1. Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for 

non-threaded applications, as unsynchronized Objects typically perform better than 

synchronized ones.


2. Hashtable does not allow null keys or values. HashMap allows one null key and any 

number of null values.


3. One of the HashMap's subclasses is LinkedHashMap, so in the event if we want to 

predict the iteration order(which is insertion order by default), we can easily change 

our HashMap to LinkedHashMap. This is very difficult to achieve with Hashtable.


If synchronization is not an issue, then use HashMap.

If synchronization is an issue then use ConcurrentHashMap.



How to make a HashMap thread-safe ?
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use Collections.synchronizedMap()



NOTE
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Even iterating over a Hashtable's entries or HashMap obtained by 

Collections.synchronizedMap is not thread safe unless we also guard the Map from being 

modified through additional synchronization.