如何使用基于“外部"的比较器创建 TreeMap价值观 [英] How to create a TreeMap with a comparator based on "external" values

问题描述

我有一个 MapotherMap 将字符串的属性映射到一些相关的整数值.

I have a Map<String, Integer> otherMap which maps properties of strings to some associated integer values.

现在我想要一个 TreeMap，它根据 otherMap 中的关联整数对键进行排序.

Now I would like a TreeMap<String, String> that orders the keys according to the associated integers in otherMap.

我应该如何解决这个问题，需要注意什么?

How should I tackle this problem, and what's important to keep in mind?

^{(这是对这个问题的跟进.)上>}

^{(This is a follow-up on this question.)}

推荐答案

在编写比较器时，重要的是要确保结果一致(即随着时间的推移相同)并且它实现了 总订单.

When writing a comparator it's important to ensure that the results are consistent (i.e. are the same over time) and that it implements a total order.

在 TreeMap 中使用比较器时，还要求它与 equals 一致，这意味着 c.compare(e1, e2) 返回 0 当且仅当 e1.equals(e2).

When using the comparator in a TreeMap it is also required that it is consistent with equals, which means that c.compare(e1, e2) return 0 if and only if e1.equals(e2).

考虑到这一点，可以按如下方式实现正确的比较器:

With this in mind, a correct comparator could be implemented as follows:

class MyComparator implements Comparator<String> {

    Map<String, Integer> otherMap;

    public MyComparator(Map<String, Integer> otherMap) {
        this.otherMap = otherMap;
    }

    @Override
    public int compare(String o1, String o2) {
        int primary = otherMap.get(o1).compareTo(otherMap.get(o2));
        if (primary != 0)
            return primary;
        // Fall back on comparing the string keys to ensure consistent results
        return o1.compareTo(o2);
    }
}

(需要注意的是，otherMap 在传递给 MyComparator 后永远不会改变也是很重要的.)

(It should be noted that it is also important that otherMap never changes after it is passed to MyComparator.)

在 Java 8 中，惯用的解决方案看起来像

In Java 8, the idiomatic solution would look like

Comparator.comparing(k -> otherMap.get(k))
          .thenComparing(k -> k);

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