Java 流：收集映射为每个流元素创建两个键

Question

I have a Java stream that invokes .collect(Collectors.toMap) . Collectors.toMap accepts a keyMapper and a valueMapper functions. I'd like to create two entries for each stream element, with two different keyMapper functions, but with the same valueMapper function. Is it possible to do this in a nice stream syntax without creating a custom collector?

Of course, I could also get one map, then add another set of keys with the same values to it, outside of the stream chain calls. But I was wondering if it could be made neater...

Basically what I have is:

List<A> someObjects = ...; // obtain somehow
Map<String, B> res = someObjects.stream().collect(Collectors.toMap(keyMapper1, valueMapper));

And functions keyMapper1 and keyMapper2 produce different strings, and I want both of those in my map with the same value.

What I can do is:

Map<A, B> map = someObjects.stream().collect(Collectors.toMap(Function.identity(), valueMapper));
Map<String, B> result = new HashMap<>();
map.forEach((a, b) -> {
    result.put(keyMapper1(a), b);
    result.put(keyMapper2(a), b);
});

But maybe something could be done without creating an intermediate variable?

Answer 1

You can use flatMap to create a stream of all the map entries first, and then collect them to a map. Something like this:

Map<String, String> map = someObjects.stream()
    .flatMap(obj -> Stream.of(
            Map.entry(keyMapper1(obj), valueMapper(obj)),
            Map.entry(keyMapper2(obj), valueMapper(obj))))
    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));