CODEWITHSUNDEEP
eclipselambdajava-8
Can't Tell
Can't Tell

Reputation: 13476

Why won't this java 8 code compile?

public static Stream<Cell> streamCells(int rows, int cols) {
    return IntStream.range(0, rows).mapToObj(row -> IntStream.range(0, cols).mapToObj(col -> new Cell(row, col)));
}

I'm using Eclipse. The following error is given by eclipse.

Type mismatch: cannot convert from Stream<Object> to Stream<ProcessArray.Cell>

Upvotes: 1

Views: 1427

Answers (3)

flo
flo

Reputation: 10291

Your Code maps to a Stream of Streams. Deviding it into declaration and return provides the following code:

Stream<Stream<Cell>> mapToObj = IntStream.range(0, rows).mapToObj(row -> IntStream.range(0, cols).mapToObj(col -> new Cell(row, col)));
return mapToObj;

You need to reduce your Streams to a single Stream:

// CAVE: poor performance
return IntStream.range(0, rows)
    .mapToObj(row -> IntStream.range(0, cols).mapToObj(col -> new Cell(row, col)))
    .reduce(Stream.empty(), Stream::concat);

Edit: As pointed out by Holger in the comments, reducing a Stream of Streams with Stream.concat() is not very performant. Use one of the other solutions utilizing the flatMap method instead of reduce.

Upvotes: 1

Can&#39;t Tell
Can&#39;t Tell

Reputation: 13476

An alternative solution to @flo's

public static Stream<Cell> streamCells(int rows, int cols) {
    return IntStream.range(0, rows).boxed()
            .flatMap(row -> IntStream.range(0, cols).mapToObj(col -> new Cell(row, col)));
}

Upvotes: 3

srborlongan
srborlongan

Reputation: 4579

@flo's answer, reworked to use flatMap instead (flatMap "embeds" the Stream returned by the flatMapping function into the original stream):

return IntStream.range(0, rows)
  .mapToObj(row -> IntStream.range(0, cols)
    .mapToObj(col -> new Cell(row, col))
  ) // int -> Stream<Cell>
  .flatmap(Function.identity()) // Stream<Cell> -> Cell
;

Upvotes: 1

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