Holger had the best answer in the comment section in my opinion:
This is a known limitation of Java 8’s type inference: it doesn’t work with chained method invocations like
genericFactoryMethod().build()
.
Thanks! About my API, I will specify the functions before using them as arguments, like this:
Function<Entry<String, Set<String>>, String> keyMapper = Entry::getKey;
Function<Entry<String, Set<String>>, Stream<String>> valueMapper = entry -> entry.getValue().stream();
EDIT: I redesigned the API thanks to Holger’s comments (thanks again!). It keeps the original element instead of a key, along with the flattened value.
public static <T, R> Function<? super T, Stream<FlatEntry<T, R>>> flatEntryMapper(Function<? super T, ? extends Stream<? extends R>> mapper)
{
return element -> mapper.apply(element).map(value -> new FlatEntry<>(element, value));
}
public static class FlatEntry<E, V>
{
/** The original stream element */
public final E element;
/** The flattened value */
public final V value;
private FlatEntry (E element, V value)
{
this.element = element;
this.value = value;
}
}
It is chainable, starting with level 2 the mapper has to process a FlatEntry
. The usage is similar to a simple flatMap
:
Map<String, Map<String, Map<String, Set<String>>>> level3Map;
// gives a stream of all the flattened values
level3Map.entrySet()
.stream()
.flatMap(entry -> entry.getValue().entrySet().stream())
.flatMap(entry -> entry.getValue().entrySet().stream())
.flatMap(entry -> entry.getValue().stream());
// gives a stream of FlatEntries with flattened values and all their original elements in nested FlatEntries
level3Map.entrySet()
.stream()
.flatMap(GdkStreams.flatEntryMapper(entry -> entry.getValue().entrySet().stream()))
.flatMap(GdkStreams.flatEntryMapper(flatEntry -> flatEntry.value.getValue().entrySet().stream()))
.flatMap(GdkStreams.flatEntryMapper(flatEntry -> flatEntry.value.getValue().stream()));