Java Lambda stream into different collections

Question

I have a Java lambda stream that parses a file and stores the results into a collection, based on some basic filtering.

I'm just learning lambdas so bear with me here if this is ridiculously bad. But please feel free to point out my mistakes.

For a given file:

#ignored
this
is
#ignored
working
fine

The code:

List<String> matches;

Stream<String> g = Files.lines(Paths.get(givenFile));

matches = g.filter(line -> !line.startsWith("#"))
           .collect(Collectors.toList());

["this", "is", "working", "fine"]

Now, how would I go about collecting the ignored lines into a second list within this same stream? Something like:

List<String> matches;
List<String> ignored; // to store lines that start with #

Stream<String> g = Files.lines(Paths.get(exclusionFile.toURI()));

matches = g.filter(line -> !line.startsWith("#"))
           // how can I add a condition to throw these
           // non-matching lines into the ignored collection?
           .collect(Collectors.toList());

I realize it would be pretty trivial to open a new stream, alter the logic a bit, and .collect() the ignored lines easily enough. But I don't want to have to loop through this file twice if I can do it all in one stream.

Answer 1

Instead of two streams you can use partitioningBy in Collector

List<String> strings = Arrays.asList("#ignored", "this", "is", "#ignored", "working", "fine");
Map<Boolean, List<String>> map = strings.stream().collect(Collectors.partitioningBy(s -> s.startsWith("#")));
System.out.println(map);

output

{false=[this, is, working, fine], true=[#ignored, #ignored]}

here I used key as Boolean but you can change it to a meaningful string or enum

EDIT

If the strings can starts with some other special characters you could use groupingBy

    List<String> strings = Arrays.asList("#ignored", "this", "is", "#ignored", "working", "fine", "!Someother", "*star");
    Function<String, String> classifier = s -> {
        if (s.matches("^[!@#$%^&*]{1}.*")) {
            return Character.toString(s.charAt(0));
        } else {
            return "others";
        }
    };
    Map<String, List<String>> maps = strings.stream().collect(Collectors.groupingBy(classifier));
    System.out.println(maps);

Output

{!=[!Someother], #=[#ignored, #ignored], *=[*star], others=[this, is, working, fine]}

also you can nest groupingBy and partitioningBy