为什么将ConcurrentNavigableMap实现为跳过列表？

Question

最近我遇到了ConcurrentSkipListMap，这是skip list ConcurrentNavigableMap的实施。现在我想知道，为什么它被实现为skip list。

skip list是并发可导航地图的“标准”实施吗？是什么让skip list特别有用？

Answer 1

ConcurrentSkipListMap源代码记录了原因。

采用这种方法而不是通常的基于数组的

有两个原因

1. 基于数组的实现似乎遇到了更多的复杂性 开销
2. 我们可以为频繁遍历的索引列表使用更便宜的算法 可以用于基本列表

这是javadoc

 /*
 * This class implements a tree-like two-dimensionally linked skip
 * list in which the index levels are represented in separate
 * nodes from the base nodes holding data.  **There are two reasons
 * for taking this approach instead of the usual array-based**
 * structure: 1) Array based implementations seem to encounter
 * more complexity and overhead 2) We can use cheaper algorithms
 * for the heavily-traversed index lists than can be used for the
 * base lists.  Here's a picture of some of the basics for a
 * possible list with 2 levels of index:
 *
 * Head nodes          Index nodes
 * +-+    right        +-+                      +-+
 * |2|---------------->| |--------------------->| |->null
 * +-+                 +-+                      +-+
 *  | down              |                        |
 *  v                   v                        v
 * +-+            +-+  +-+       +-+            +-+       +-+
 * |1|----------->| |->| |------>| |----------->| |------>| |->null
 * +-+            +-+  +-+       +-+            +-+       +-+
 *  v              |    |         |              |         |
 * Nodes  next     v    v         v              v         v
 * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+
 * | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null
 * +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+  +-+