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.

it must be something like g.filter(..).map(t->t::toString)).collect(..toList()) — user6601906
– user6601906, Commented Oct 13, 2016 at 3:40
See the final example in the Oracle docs: docs.oracle.com/javase/8/docs/api/java/util/stream/… — Gene
– Gene, Commented Oct 13, 2016 at 3:53

Saravana · Accepted Answer · 2016-10-13 04:17:21Z

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

Will Hayworth · Accepted Answer · 2016-10-13 04:01:24Z

1

I think the closest you could come to a generic approach for this would be something like peek:

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

I mention it because unlike with the partitioning Collector you could, at least in theory, change together however many peeks you want--but, as you can see, you have to duplicate logic, so it's not ideal.

answered Oct 13, 2016 at 4:01

Will Hayworth

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