Maps

• Maps associate the data elements in the collection with keys. The keys are derived from type Object, and may be different a type than the data. The get and put methods provide access to elements at a particular key.

     Map userAges = new HashMap();
  
     ages.put("John", new Integer(25));
     ages.put("Nicole", new Integer(32));
     ages.put("Bob", new Integer(43));
  
     // How old is Bob?
     Integer bobsAge = (Integer)userAges.get("Bob");
  
     // if we have a user named Nicole, what's her age?
     if(userAges.containsKey("Nicole"))
     {
      Integer nicolesAge = (Integer)userAges.get("Nicole");
     }
  

• Caution: If the key object is mutable, take care not to change its value after using it in a Map.
• There are several Map implementations including HashMap and Hashtable.
• As a Java 1.1 legacy class, Hashtable is syncronized for thread safety.