一提到Map，我們最常想到的是HashMap。在實際coding過程中，陸續(xù)遇到其他的一些map implementation，想在此分享：

LinkedHashMap

IdentityHashMap: Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map.
Note that this implementation is not synchronized.

簡言之，和HashMap比較來說，LinkedHashMap能夠保證entry的順序。

IdentityHashMap

IdentityHashMap: This class implements the Map interface with a hash table, using reference-equality in place of object-equality when comparing keys (and values). In other words, in an IdentityHashMap, two keys k1 and k2 are considered equal if and only if (k1==k2). (In normal Map implementations (like HashMap) two keys k1 and k2 are considered equal if and only if (k1==null ? k2==null : k1.equals(k2)).)
Note that this implementation is not synchronized.

簡言之，對于key和value，HashMap用equals比較大小，而IdentityHashMap用 == 比較大小，比HashMap更快。

Multimap

Multimap: A collection that maps keys to values, similar to Map, but in which each key may be associated with multiple values. You can visualize the contents of a multimap either as a map from keys to nonempty collections of values:

• a → 1, 2
• b → 3
  ... or as a single "flattened" collection of key-value pairs:
• a → 1
• a → 2
• b → 3

每個key可以對應多個values。

Tips
以引用里的key, values為例：

• myMultimap.values() collection is [1, 2, 3], not [[1, 2], [3]].
• myMultimap.size() is 3, not 2.

Map.Entry<K, V>

Map.Entry<K, V>是一個interface。它的implementation包括：AbstractMap.SimpleEntry<K,V>
AbstractMap.SimpleImmutableEntry<K,V>
正因為Map.Entry<K, V>是interface，我們不能instantiate它（比如new）。

To be continue...