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?
Best Answer
The lambda expression in
flatMap
needs to return aStream
, as can be seen by the argument offlatMap
which is of typeFunction<? super T, ? extends Stream<? extends R>>
.The following code will compile and run fine:
because the lambda expression
str -> duplicate(str).stream()
is of typeFunction<String, Stream<String>>
.