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.