can I define a new Comparator using Comparator.comparing() when the comparison is based on the result of my own method?

Question

I want to create a Comparator that compares between Files, based on a string that is created by a method I built (that takes a File).

"getType" is my own function (and I can't use a different class for files other than File). I've tried this:

private static Comparator<File> typeComparator = (File file1, File file2)-> getType(file1.getName()).compareTo(getType(file2.getName()));

but IntelliJ won't take it, saying it should be replaced with Comparator.comparing. I don't understand how to do this, because to my understanding, in Comparator.comparing I can only use the attributes of a File, and not my own function.

  private static String getType(String fileName) {
        int i = fileName.lastIndexOf('.');
        if (i > 0) {
            return fileName.substring(i+1);
        }
        return "";
    }

I would like to implement the comparison using Comparator.comparing, if possible, or a different way that is better than mine.

Answer 1

The function you give to Comparator.comparing needs to map the type you sort to an already sortable type. In your case, the original type is File , and the sortable type is String (the result of your getType() . All you need to do is map File -> String to get the filename, then once more to get the type.

IE

Comparator.comparing(f -> getType(f.getName());

Answer 2

You can do:

Comparator.comparing((file) -> getType(file.getName()))

Alternatively, you can change the getType() method to accept a file instead, so you can write

Comparator.comparing(this::getType)

(and yes, IntelliJ is usually able to rewrite your code automatically if you ask by pressing alt+enter)