How to use loop in lambda functions in JAVA?

Question

I want to create a FileNameFilter using a lambda function.

My main problem is that I want to introduce a loop inside the function. I tried using filters but I am unable to create filters based on external list.

Following is my code:

    File targetDir = new File(path);
    FilenameFilter deleteFilter = new FilenameFilter() {
        @Override
        public boolean accept(File dir, String name) {
            for(String tag: pageTagsList){
                if(name.equals(tag))
                    return true;
            }
            return false;
        }
    };
    File[] foundFiles = targetDir.listFiles(deleteFilter);

What I want to do is something like this:

File[] foundFiles = targetDir.listFiles(
            (dir,name)->pageTagsList.stream()
            .filter(//code to filter here)
            .orElse(false)
            );

Is there a correct way to do this?

Maybe this is a contrived example and the point is to know how to iterate within a lambda body in general, but if not, the best solution would be to use a pageTagsSet of type Set<String>. Looking up in a HashSet is way faster than iterating over a list. So (dir,name)->pageTagsList.contains(name) would do it — Hank D
– Hank D, Commented Oct 20, 2016 at 23:07

Aaron Davis · Accepted Answer · 2016-10-19 07:54:44Z

6

You can have a lambda be any code block (though any variables referenced from inside of it must be final). For your desired outcome you can do something like the following:

File[] foundFiles = targetDir.listFiles((dir, name) -> pageTagsList.stream().anyMatch(name::equals));

answered Oct 19, 2016 at 7:54

Aaron Davis

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Comments

marstran · Accepted Answer · 2016-10-19 07:50:00Z

2

A lambda can contain a block of code. The syntax is like this:

(params) -> {
    // Some code.

    for (Element elem : elements) {
        // Some code using elem.
    }

    return returnValue;
}

answered Oct 19, 2016 at 7:50

marstran

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Comments

xenteros · Accepted Answer · 2016-10-19 07:52:10Z

1

List<String> tags = pageTagList
                   .stream()
                   .filter(t -> t.equals(name)).collect(Collectors.toList());

If you need explanation of anything above, just ask in comment. I find this code self explaining.

answered Oct 19, 2016 at 7:52

xenteros

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user4046031 · Accepted Answer · 2016-10-19 08:21:51Z

1

@Override
public boolean accept(File dir, String name) {    
    return pageTagsList.stream()
        .filter(name::equals)
        .findAny()
        .isPresent()
}

This is just showcase of returning true/false, answer from Aaron Davis is most elegant

answered Oct 19, 2016 at 8:21

user4046031

1 Comment

Holger Over a year ago

Actually, the most elegant solution is pageTagsList.contains(name). Since Java 2…

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