Collections in Java can refer to one of the below 3 things:

  • Data structure to store information
  • Collection interface
  • Collections utility class containing various static methods for different operations

Most commonly, by collection, people refer to the first one, i.e, data structure to store information. Different collections in Java are:

Type Common collections Not so common
Lists ArrayList, LinkedList Vector
Maps HashMap, LinkedHashMap, TreeMap Hashtable, ConcurrentHashMap
Sets HashSet, LinkedHashSet, TreeSet  

Hashtable is just like HashMap except that all methods are simply synchronized. Whereas in ConcurrentHashMap, the read isn’t blocked. However, both are thread-safe.