Java 8 - Type mismatch: cannot convert from List<Serializable> to List<String>

Question

I have a list of Strings:

List<String> list = Arrays.asList("a1,a2", "b1,b2");

Then to convert everything in a list like: "a1","a2","b1","b2" wrote this:

List<String> ss1 = list.stream()
                       .flatMap(s -> Stream.of(s.split(",")))
                       .collect(Collectors.toList());

But I had an error: "Type mismatch: cannot convert from List<Serializable> to List<String>". I handled the problem changing into this:

List<String> ss2 = list.stream()
                       .flatMap(s -> Arrays.stream(s.split(",")))
                       .collect(Collectors.toList());

Eclipse Neon suggests that the difference is in the flatMap return type. First flatMap returns a List<Serializable> second returns a List<String>.

But both Stream.of() and Arrays.stream() returns a <T> Stream<T> (Eclipse suggests that they both returns a Stream<String>).

And again, Stream.of() internally use (and returns the output of) Arrays.stream(). So, again, what's wrong in the first case?

Answer 1

This is an Eclipse bug

_{Bug 508834, thanks to @Tunaki}

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Notice the method signatures:

//Stream.of
<T> Stream<T> of(T... values)
//Arrays.stream
<T> Stream<T> stream(T[] array)

Now, for Arrays.stream it is obvious that a call with an array of type T will return a Stream<T>. But with Stream.of should it return Stream<T> or Stream<T[]>? i.e. what is the type of the varags; are you passing your array are the first parameter (so the varargs are an array of arrays) or are you passing your array as all the parameters?

That's your issue.