带有内部列表的列表以映射 Java Stream Api

Question

I have a list of elements:

List<OuterElement> outerElements;

OuterElement object has 2 property:

String key;

List<InnerElement> innerElements;

InnerElement object has 1 property:

String value;

I need to create a map where for each innerElement will be created an Entry, where key will be outerElement key, and value will be innerElement value;

Map<String, String> // actually Map<OuterElement.key, InnerElement.value> 

How to do it using Stream API?

I have tried something like this, but it doesn't work:

       Map<String, String> result = outerElements.stream()
        .forEach(outerElement -> outerElement.getInnerElements().stream().collect(Collectors.toMap(OuterElement::getKey, InnerElement::getValue));

Answer 1

In addition to Ruslan answer for java 8 you could use this one:

    Map<String, List<String>> collect = elements.stream()
        .collect(Collectors.groupingBy(OuterElement::getKey,
            Collectors.collectingAndThen(Collectors.toList(), outers -> outers.stream()
                .flatMap(o -> o.getInnerElements().stream())
                .map(InnerElement::getValue)
                .collect(Collectors.toList()))));

if you could use something like MultiValueMap :

    MultiValueMap<String, String> map = elements.stream()
        .collect(Collectors.groupingBy(OuterElement::getKey,
            LinkedMultiValueMap::new,
            Collectors.collectingAndThen(Collectors.toList(), outers -> outers.stream()
                .flatMap(o -> o.getInnerElements().stream())
                .map(InnerElement::getValue)
                .collect(Collectors.toList()))));