Getting the result of a Lambda in java

Question

I was wondering how to refer to the result of a lambda in Java? This is so I can store the results into an ArrayList, and then use that for whatever in the future.

The lambda I have is:

try {
    Files.newDirectoryStream(Paths.get("."),path -> path.toString().endsWith(".txt"))
         .forEach(System.out::println);
} catch (IOException e) {
    e.printStackTrace();
}

and inside the .forEach() I want to be able to assign each file name to the array in turn, for example, .forEach(MyArrayList.add(this))

Thanks for any help in advance!

Answer 1

Use :

List<String> myPaths = new ArrayList<>();
Files.newDirectoryStream(Paths.get("."), path -> path.toString().endsWith(".txt"))
     .forEach(e -> myPaths.add(e.toString()));

Edit :

We can achieve the same in one line using :

List<String> myPaths = Files.list(Paths.get("."))
                            .filter(p -> p.toString().endsWith(".txt"))
                            .map(Object::toString)
                            .collect(Collectors.toList());

Answer 2

You can achieve that by collecting the result of the newDirectoryStream operation :

1. You can add the elements in a list as you iterate it but that's not the better way :

   List<Path> listA = new ArrayList<>();
   Files.newDirectoryStream(Paths.get(""), path -> path.toString().endsWith(".txt"))
        .forEach(listA::add);
   

2. You may use another method like find which returns a Stream<Path> that would be easier to use and collect the elements in a list :

   List<Path> listB = Files.find(Paths.get(""), 1,(p, b) -> p.toString().endsWith(".txt"))
                           .collect(Collectors.toList());
   

3. Or Files.list()

   List<Path> listC = Files.list(Paths.get("")).filter(p -> p.toString().endsWith(".txt"))
                           .collect(Collectors.toList());
   

Answer 3

You can create a variable which represents the current element in forEach and refer it, for example:

ArrayList<Path> paths = new ArrayList<>();

Files.newDirectoryStream(Paths.get("."), path -> path.toString().endsWith(".txt"))
        .forEach(path -> paths.add(path));

which also can be simplied to:

Files.newDirectoryStream(Paths.get("."), path -> path.toString().endsWith(".txt"))
        .forEach(paths::add);