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!

Nicholas K · Accepted Answer · 2018-10-27 18:32:44Z

10

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());

edited Oct 27, 2018 at 18:32

answered Oct 2, 2018 at 14:01

Nicholas K

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1 Comment

Ritesh Over a year ago

Please also see Side-effects section of Package java.util.stream Description. For more efficient execution, you should replace forEach by reduction. See Holger's comment to the answer below.

azro · Accepted Answer · 2018-10-02 14:53:04Z

9

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

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);

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());

Or Files.list()

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

edited Oct 2, 2018 at 14:53

answered Oct 2, 2018 at 14:07

azro

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1 Comment

Holger Over a year ago

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

xingbin · Accepted Answer · 2018-10-02 14:09:39Z

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);

edited Oct 2, 2018 at 14:09

answered Oct 2, 2018 at 13:59

xingbin

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1 Comment

DudeDoesThings Over a year ago

Using the method reference paths::add instead of path -> paths.add(path) would be even nicer in my opinion.

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