Maps

Generalized form of an array; any object can be an index
The type used for the index is the key; the type of value being indexed is the value
Object get(Object key) returns the value for a given key
Object put(Object key, Object value) associates key with value; if key is already in use, the value replaced is returned
Maps are not Collections, strictly speaking, as they cannot be iterated directly
Set keySet() returns all of the keys of a Map as a Set, which is useful for iteration because Map does not implement Collection or Iterable
Implementations are TreeMap, HashMap, and LinkedHashMap