如何使用基于另一个列表的lambda从列表中删除元素

Question

I have List of file Paths: .

List<Path> filePaths; //e.g. [src\test\resources\file\15\54\54_exampleFile.pdf]

54 above refers to file ID

I then obtain a Set of String Ids which my application can handle as follows:

Set<String> acceptedIds = connection.getAcceptedIDs(); //e.g. elements [64, 101, 33]

How can I use Java 8 lambdas to filter out all elements in filePaths that do not contain any of the acceptable Ids that are contained in acceptedIds collection Set.

In other words, I would like to retain in filePaths only the paths that have ids which are in acceptedIds set. For example, 54 is not in the above list so is removed.

filePaths.stream().filter(...).collect(Collectors.toList());

Answer 1

The most efficient way is to extract the ID from the path, then attempt to find it in the Set, making each filter execute in constant time, ie O(1) giving an overall O(n) , where n is the number of paths:

filePaths.stream()
  .filter(p -> acceptedIds.contains(p.getParent().getFileName().toString()))
  .collect(Collectors.toList());

If the reverse approach is done, where each acceptedIds is searched for in the path (as in other answers), each filter is O(m*k) , where m is the number of acceptedIds and k is the average Path length, giving an overall O(n * m * k) , which will perform very poorly for even moderate sizes of collections.

Answer 2

You could write:

filePaths.stream()
         .filter(p -> acceptedIds.stream().anyMatch(id -> p.toString().contains(id)))
         .collect(toList());

This filters each path such that at least one of the acceptedIds is contained in the string representation of the path. You might want to implement something better than contains here, depending on your use-case (matching the beginning of the filename for example).

anyMatch is an operation that determines if at least one element matches the given predicate.

Note that this answer does not make any assumption about the path to filter out elements. If you can safely say that in each path, the parent directory is named with the id, you should definitely go with @Bohemian answer, for performance reason.

Answer 3

Like so:

List removeMissing(List l1, List l2) {
    List ret = l1.stream()
        .filter(o -> l2.contains(o)) //Keep if object o satisfies the condition "l2 contains a reference to this object"
        .collect(Collectors.toList());
    return ret;
}

Answer 4

If your file name structure is constant, I'd use a regex first to extract the number, and then will check if it is among the desired ids.

final Set<String> acceptedIds = ...
// Matches the number of the file, concluded with the underscore
final Pattern extractor = Pattern.compile("\.*(?<number>\d+)_")
filePaths.stream().filter( path -> {
    final Matcher m = extractor
        .matcher(path.getFileName().toString());
    m.find();
    return acceptedIds.contains(m.group("number"));
})
...