如何使用Java流API过滤地图? [英] How to filter a map with Java stream api?

问题描述

Map<Integer, String> map = new HashMap<>();
map.put(1, "f");
map.put(2, "i");
map.put(3, "a");
map.put(4, "c");....etc

现在我有一个列表:

List<Integer> picks = {1,3}

我想取回一个字符串列表，即映射中与"pick"列表中的键值匹配的值.因此，我希望取回{"f"，结果" .有没有一种方法可以使用 java流api 以一种优雅的方式做到这一点?

I would like to get back a list of Strings, ie, values from map that matches the key values, found in the 'pick' list.So, I am expecting to get back {"f", "a"} as result. Is there a way to use java stream api to do it in elegant way?

当有一个值时，我就是这样:

When there is one value , I am doing it this way:

map.entrySet().stream()
            .filter(entry -> "a".equals(entry.getValue()))
            .map(entry -> entry.getValue())
            .collect(Collectors.toList())

但是在要筛选的键/小贴士列表时会变得很困难.

But getting hard time when there is a list of keys/picks to filter with.

推荐答案

您可以在Stream#filter中使用List.contains()来仅接受列表中存在的那些值:

You can use List.contains() in the Stream#filter to only accept those values which are present in the list:

List<String> result = map.entrySet()
                         .stream()
                         .filter(ent -> picks.contains(ent.getKey()))
                         .map(Map.Entry::getValue)     
                         .collect(Collectors.toList());

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