Reapply "Add custom equals and hashCode for HashMap implementation."

sdk/lib/collection/hash_map.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.collection;
 
 /** Default function for equality comparison in customized HashMaps */
 bool _defaultEquals(a, b) => a == b;
 /** Default function for hash-code computation in customized HashMaps */
 int _defaultHashCode(a) => a.hashCode;
 
+/** Type of custom equality function */
+typedef bool _Equality<K>(K a, K b);
+/** Type of custom hash code function. */
+typedef int _Hasher<K>(K object);
+
 /**
  * A hash-table based implementation of [Map].
  *
  * The keys of a `HashMap` must have consistent [Object.operator==]
  * and [Object.hashCode] implementations. This means that the `==` operator
  * must define a stable equivalence relation on the keys (reflexive,
  * anti-symmetric, transitive, and consistent over time), and that `hashCode`
  * must be the same for objects that are considered equal by `==`.
  *
  * The map allows `null` as a key.
  */
-class HashMap<K, V> implements Map<K, V> {
+abstract class HashMap<K, V> implements Map<K, V> {
   /**
    * Creates a hash-table based [Map].
    *
    * The created map is not ordered in any way. When iterating the keys or
    * values, the iteration order is unspecified except that it will stay the
    * same as long as the map isn't changed.
    *
    * If [equals] is provided, it is used to compare the keys in the table with
    * new keys. If [equals] is omitted, the key's own [Object.operator==] is used
    * instead.
    *
    * Similar, if [hashCode] is provided, it is used to produce a hash value
    * for keys in order to place them in the hash table. If it is omitted, the
    * key's own [Object.hashCode] is used.
    *
    * The used `equals` and `hashCode` method should always be consistent,
    * so that if `equals(a, b)` then `hashCode(a) == hashCode(b)`. The hash
    * of an object, or what it compares equal to, should not change while the
    * object is in the table. If it does change, the result is unpredictable.
+   *
+   * It is generally the case that if you supply one of [equals] and [hashCode],
+   * you also want to supply the other. The only common exception is to pass
+   * [identical] as the equality and use the default hash code.
    */
-  factory HashMap({bool equals(K key1, K key2), int hashCode(K key)}) {
-    if (equals != null || hashCode != null) {
-      if (equals == null) {
-        equals = _defaultEquals;
-      } else if (hashCode == null) {
-        hashCode = _defaultHashCode;
-      }
-      // Create new CustomHashMap<K, V>(equals, hashCode).
-      throw new UnimplementedError("Not implemented yet");
-    }
-    return new HashMap<K, V>._internal();
-  }
-
-  /** Creates an empty `HashMap` with the default equals and hashCode. */
-  external HashMap._internal();
+  external factory HashMap({bool equals(K key1, K key2), int hashCode(K key)});
 
   /**
    * Creates a [HashMap] that contains all key value pairs of [other].
    */
   factory HashMap.from(Map<K, V> other) {
     return new HashMap<K, V>()..addAll(other);
   }
 
   /**
    * Creates a [HashMap] where the keys and values are computed from the
@@ skipping 24 matching lines @@
    * If [keys] contains the same object multiple times, the last occurrence
    * overwrites the previous value.
    *
    * It is an error if the two [Iterable]s don't have the same length.
    */
   factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
     HashMap<K, V> map = new HashMap<K, V>();
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
-
-  external int get length;
-  external bool get isEmpty;
-  external bool get isNotEmpty;
-
-  external Iterable<K> get keys;
-  external Iterable<V> get values;
-
-  external bool containsKey(Object key);
-  external bool containsValue(Object value);
-
-  external void addAll(Map<K, V> other);
-
-  external V operator [](Object key);
-  external void operator []=(K key, V value);
-
-  external V putIfAbsent(K key, V ifAbsent());
-
-  external V remove(Object key);
-  external void clear();
-
-  external void forEach(void action(K key, V value));
-
-  String toString() => Maps.mapToString(this);
 }