A Map is an associative container, mapping a key to a value. Null values are supported, but null keys are not.

abstract class Map<K, V> {
 /**
  * Creates a map with the default implementation.
  */
 factory Map() => new HashMap<K, V>();

 /**
  * Creates a [Map] that contains all key value pairs of [other].
  */
 factory Map.from(Map<K, V> other) => new HashMap<K, V>.from(other);


 /**
  * Returns whether this map contains the given [value].
  */
 bool containsValue(V value);

 /**
  * Returns whether this map contains the given [key].
  */
 bool containsKey(K key);

 /**
  * Returns the value for the given [key] or null if [key] is not
  * in the map. Because null values are supported, one should either
  * use containsKey to distinguish between an absent key and a null
  * value, or use the [putIfAbsent] method.
  */
 V operator [](K key);

 /**
  * Associates the [key] with the given [value].
  */
 void operator []=(K key, V value);

 /**
  * If [key] is not associated to a value, calls [ifAbsent] and
  * updates the map by mapping [key] to the value returned by
  * [ifAbsent]. Returns the value in the map.
  *
  * It is an error to add or remove keys from map during the call to
  * [ifAbsent].
  */
 V putIfAbsent(K key, V ifAbsent());

 /**
  * Removes the association for the given [key]. Returns the value for
  * [key] in the map or null if [key] is not in the map. Note that values
  * can be null and a returned null value does not always imply that the
  * key is absent.
  */
 V remove(K key);

 /**
  * Removes all pairs from the map.
  */
 void clear();

 /**
  * Applies [f] to each {key, value} pair of the map.
  *
  * It is an error to add or remove keys from the map during iteration.
  */
 void forEach(void f(K key, V value));

 /**
  * The keys of [this].
  */
 // TODO(floitsch): this should return a [Set].
 Iterable<K> get keys;

 /**
  * The values of [this].
  */
 Iterable<V> get values;

 /**
  * The number of {key, value} pairs in the map.
  */
 int get length;

 /**
  * Returns true if there is no {key, value} pair in the map.
  */
 bool get isEmpty;
}