被 Java8 Collectors.toMap 弄糊涂了 [英] Confused by Java8 Collectors.toMap

问题描述

我有一个如下所示的集合，我想过滤掉除非月末日期之外的所有内容.

I have a collection that looks like below, and I want to filter out the everything except the dates that aren't the end of the months.

2010-01-01=2100.00, 
2010-01-31=2108.74, 
2010-02-01=2208.74, 
2010-02-28=2217.92, 
2010-03-01=2317.92, 
2010-03-31=2327.57, 
2010-04-01=2427.57, 
2010-04-30=2437.67, 
2010-05-01=2537.67, 
2010-05-31=2548.22, 
2010-06-01=2648.22, 
2010-06-30=2659.24, 
2010-07-01=2759.24, 
2010-07-31=2770.72, 
2010-08-01=2870.72, 
2010-08-31=2882.66, 
2010-09-01=2982.66, 
2010-09-30=2995.07, 
2010-10-01=3095.07, 
2010-10-31=3107.94, 
2010-11-01=3207.94, 
2010-11-30=3221.29

我有以下过滤条件.frequency.getEnd 返回与给定 LocalDate 的月末匹配的 LocalDate.

I have the following filter criteria. frequency.getEnd returns a LocalDate matching the end of the month for the given LocalDate.

.filter(p -> frequency.getEnd(p.getKey()) == p.getKey())

所以现在我想我必须将这个过滤后的流转换回地图.我想我使用收集器来做到这一点.因此我补充说:

So now I think I have to converted this filtered stream back to a map. And I think I use a collector to do that. Thus I add:

.collect(Collectors.toMap(/* HUH? */));

但我不知道如何处理 Collectors.toMap.阅读示例让我感到困惑.这是我当前的代码，显然不起作用.

But I don't know what to do with Collectors.toMap. Reading examples leaves me confused. Here's my current code which obviously doesn't work.

TreeMap<LocalDate, BigDecimal> values = values.entrySet()
                                              .stream()
                                              .filter(p -> frequency.getEnd(p.getKey()) == p.getKey())
                                              .collect(Collectors.toMap(/* HUH? */));

推荐答案

除了前面的答案注意，如果你不需要保留原始地图，你可以在不使用 Stream API 的情况下就地执行这样的过滤:

In addition to the previous answers note that if you don't need to keep the original map, you can perform such filtering in-place without using the Stream API:

values.keySet().removeIf(k -> !frequency.getEnd(k).equals(k));

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