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Consider this piece of code:

private static ArrayList<Level> levels = new ArrayList<>();

static {
    try (Stream<Path> paths = Files.walk(Paths.get("levels"))) {
        paths
            .filter(Files::isRegularFile)
            .forEach(levels.add(new Level(file))); //CAN'T DO!!
    } catch (IOException e) {
        e.printStackTrace();
    } 
}

I think the code pretty much says what I'm trying to do. I found this piece of code somewhere and I tried to apply it, to create Level objects from folder with files such as level1.txt, level2.txt, etc. The Level class takes a File as argument.

The forEach method gives out a SyntaxError.

  1. What are the "::" in the filter method?
  2. Why is the path followed by a new line and 2 methods? I've never seen such a thing before.
  3. How can I make this work properly?
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  • 1
    "This code does not work." Please supply an adequate error description, including a full copy of the error and edit it into the question. Commented Apr 26, 2020 at 21:19
  • 1
    1 may google it (stackoverflow.com/questions/20001427/…). 2 just return line, same as paths.filter(Files::isRegularFile).forEach(levels.add(new Level(file))); which is Stream (chaining method here) Commented Apr 26, 2020 at 21:20
  • You should put a lambda like (file) -> levels.add(new Level(file)) into the forEach instead of an expression Commented Apr 29, 2020 at 14:43
  • I was expecting the complete error message, not just syntax error Commented Jul 21 at 14:27

1 Answer 1

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  1. The following explains well what is :: (double colon) operator since Java 8

  2. Code can be run written on several lines, but this is same as the following. Your calling the method filter then calling forEach on the object returned by filter

    paths.filter(Files::isRegularFile).forEach(levels.add(new Level(file)));

  3. Get it work, you need to define file variable this is done with lambda functions

    paths.filter(Files::isRegularFile).forEach(file -> levels.add(new Level(file)));

NOT POSSIBLE because walks() throws IOException

In case you don't need the try/catch you can use Collectors to directly build the list

private static ArrayList<Level> levels = Files.walk(Paths.get("levels"))
                                              .filter(Files::isRegularFile)
                                              .map(file -> new Level(file))
                                              .collect(Collectors.toList());
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5 Comments

The code does not work. I get SyntaxError in levels.add(new Level(file).
@DanielOscar I got none; So give the exact detail of the error and the constructor of Level class
The Level constructor just takes a File objects as parameter.
IDE suggest I change constructor of Level to receive a Path object instead
@DanielOscar walk gives out instances of Path not File, so you may adapt, use Path or get the file behind the path

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