使Multimap仅在Java中按键排序 [英] Having a Multimap sorted on keys only in Java

问题描述

我想要一个仅基于键排序的 c.g.c.c.Multimap 。不应对值进行排序。我试图用guava的 TreeMultimap 构建一些东西，但我不能使用它，因为值类型没有实现 Comparable 。

I would like to have a c.g.c.c.Multimap that is sorted based on keys only. The values shouldn't be sorted. I've tried to build something with guava's TreeMultimap, but I can't use it because the value type doesn't implement Comparable.

public class MyObject /* doesn't implement Comparable */ {
  private String name;
  private int score;
  // Getters/setters are implemented
  public static Function<MyObject,Integer> myObjectToScore {
    @Override public Integer apply (MyObject o) { return o.score; }
  }
  public static Multimap<Integer,MyObject> indexOnScore(Iterable<MyObject> i) {
    Multimap<Integer,MyObject> m = Multimaps.index(i, myObjectToScore());
    // Do the sort of the keys.
    return m;
  }
}

我考虑过获得 SortedSet 的键，然后遍历排序集中的每个键以获取各种值，但我希望在Guava中使用现有的（但未发现的）功能而不是使用这种 hack 。

I've thought about getting a SortedSet of the keys, then iterating over each of these keys in the sorted set to fetch the various values, but I was hoping using an existing (yet undiscovered) feature in Guava rather than using this kind of hack.

注意：我不会让 MyObject 实现可比较因为它对我的实际对象毫无意义。

Note: I won't make MyObject implement Comparable because it makes no sense with my actual object.

输入示例/ output：

Example of input/output:

Set<MyObject> s = Sets.newHashSet(
  new MyObject("a", 2),
  new MyObject("b", 3),
  new MyObject("c", 1),
  new MyObject("d", 3),
  new MyObject("e", 1)
); // Assuming constructor MyObject(String name, int score)

for (Map.Entry<Integer, MyObject> e: MyObject.indexedOnScore(s).entries()) {
  System.out.printf("%d -> %s%n", e.getKey(), e.getValue().getName());
}

打印：

1 -> c // or switched with line below
1 -> e
2 -> a
3 -> b // or switched with line below
3 -> d

推荐答案

Multimaps.index 返回 ImmutableListMultimap ，因此在创建它之后将无法对其进行排序。但是，您可以先创建 Iterable< MyObject> 的排序副本，然后将其输入 Multimap.index 。 .. ImmutableListMultimap 保持事物的顺序与它们相同。

Multimaps.index returns an ImmutableListMultimap, so you wouldn't be able to sort it after creating it. You could, however, first create a sorted copy of your Iterable<MyObject> and feed that to Multimap.index... ImmutableListMultimap keeps things in the same order it was given them.

public static ImmutableMultimap<Integer, MyObject> indexOnScore(Iterable<MyObject> i) {
  List<MyObject> sorted = Ordering.natural().onResultOf(myObjectToScore())
      .sortedCopy(i);
  return Multimaps.index(sorted, myObjectToScore());
}

另一种选择可能是创建 TreeMultimap 并使用 Ordering.arbitrary（）作为值的比较器。

Another option might be to create a TreeMultimap and use Ordering.arbitrary() as the Comparator for the values.

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