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What are the differences between a HashMap and a Hashtable in Java

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In this tutorial we are going to understand the differences between HashMap and a Hashtable in Java.

There are several differences between HashMap and Hashtable in Java:

  • Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
  • Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
  • One of HashMap’s subclasses is LinkedHashMap, so in the event that you’d want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn’t be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I’d recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

Note on Some Important Terms:

  • Synchronized means only one thread can modify a hash table at one point in time. Basically, it means that any thread before performing an update on a Hashtable will have to acquire a lock on the object while others will wait for the lock to be released.
  • Fail-safe is relevant within the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object “structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke the set method since it doesn’t modify the collection “structurally”. However, if prior to calling set, the collection has been modified structurally, IllegalArgumentException will be thrown.
  • Structurally modification means deleting or inserting element which could effectively change the structure of the map.