Why is this usage of Stream::flatMap wrong?
flatmap, java, java-8, java-stream
Solution
The lambda expression in `flatMap` needs to return a `Stream`, as can be seen by the argument of `flatMap` which is of type `Function<? super T, ? extends Stream<? extends R>>`.
The following code will compile and run fine:
listOfStrings.stream()
.flatMap(str -> duplicate(str).stream()) // note the .stream() here
.collect(Collectors.toList());
because the lambda expression `str -> duplicate(str).stream()` is of type `Function<String, Stream<String>>`.
Problem
I expected to be able to use Stream::flatMap like this ``` public static List<String> duplicate(String s) { List<String> l = new ArrayList<String>(); l.add(s); l.add(s); return l; } listOfStrings.stream().flatMap(str -> duplicate(str)).collect(Collectors.toList()); ``` But I get the following compiler error Test.java:25: error: incompatible types: cannot infer type-variable(s) R listOfStrings.stream().flatMap(str -> duplicate(str)).collect(Collectors.toList()); (argument mismatch; bad return type in lambda expression List cannot be converted to Stream) where R,T are type-variables: R extends Object declared in method flatMap(Function>) T extends Object declared in interface Stream In scala I can do what I believe to be equivalent ``` scala> List(1,2,3).flatMap(duplicate(_)) res0: List[Int] = List(1, 1, 2, 2, 3, 3) ``` Why is this not a valid usage of flatMap in java?