set元素上的flatMap

Question

我有一个Map<String, Set<String>>，可以说是{"a": {"a1", "a2", "a3"}, "b": {"b1", "b2", "b3"}, "c": {"c1", "c2"}, "d": {}}

我有一组地图键，并且我想将流集的每个元素平面映射到我地图中对应值集的元素，例如

输入流：

{"a","b"}
{"a","c"}
{"b","c","d"}

输出流：

//first set
{"a1","b1"}
{"a1","b2"}
{"a1","b3"}
{"a2","b1"}
{"a2","b2"}
{"a2","b3"}
{"a3","b1"}
{"a3","b2"}
{"a3","b3"}
//second set
{"a1","c1"}
{"a1","c2"}
{"a2","c1"}
{"a2","c2"}
{"a3","c1"}
{"a3","c2"}
//third set would be flatmapped to nothing, as "d" is mapped to an empty set

如何使用Java8流来做到这一点？

仅使用Java SE 8 API是否有更好的方法？

Answer 1

您可以使用Apache Commons Collections

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.4</version>
</dependency>

示例：
public class App {

    public static void main(String[] args) {
        // With an ArrayList for the values
        MultiValuedMap<String, String> map1 = new ArrayListValuedHashMap<>();
        map1.put("a", "a1");
        map1.put("a", "a2");
        map1.put("a", "a3");
        map1.put("a", "a4");
        map1.put("a", "a5");
        map1.put("b", "b1");
        map1.put("b", "b1");
        map1.put("b", "b3");
        map1.put("b", "b4");
        map1.put("b", "b5");
        map1.entries().forEach(e -> System.out.println(e.getKey() + " - " + e.getValue()));
        System.out.println("----");
        // With a HashSet for the values
        MultiValuedMap<String, String> map2 = new HashSetValuedHashMap<>(map1);
        map2.entries().forEach(e -> System.out.println(e.getKey() + " - " + e.getValue()));
    }
}

输出看起来像这样：
a - a1
a - a2
a - a3
a - a4
a - a5
b - b1
b - b1
b - b3
b - b4
b - b5
----
a - a1
a - a2
a - a3
a - a4
a - a5
b - b3
b - b4
b - b5
b - b1