ConcurrentHashMap通过稍微地松弛它对调用者的承诺而获得了更高的并发性。
ConcurrentHashMap achieves higher concurrency by slightly relaxing the promises it makes to callers.
调用者并不知道这些实现机制，它仍然像以前那样接收一个包含英雄数据的承诺。
The caller is unaware of these machinations. It receives a Promise of heroes just as it did before.
问承诺：在相同的顺序调用回调注册吗？
Q promise: are callbacks invoked in the same order as registered?
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