java diferencia hashmap hashtable

/**
 * There are several differences between HashMap and Hashtable in Java:
 * 
 * 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 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.
 */
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