Tukukkk

I am trying to convert the following text input file:

A=groupA1

A=groupA2

A=groupA3

B=groupB1

B=groupB2

into Map<String, List<String>> by splitting each line on "="

So far I manged to get this sort of output:

KEY: A

VALUE: A=groupA1

VALUE: A=groupA2

VALUE: A=groupA3

KEY: B

VALUE: B=groupB1

VALUE: B=groupB2

using such code:

File reqFile = new File("test.config");



try (Stream<String> stream = Files.lines(reqFile.toPath())) {

    Map<String, List<String>> conf = stream.collect(Collectors.groupingBy(s -> s.split("=")[0]));

    for (Map.Entry<String, List<String>> entry: conf.entrySet()) {

        System.out.println("KEY: " + entry.getKey());

        for (String value : entry.getValue()) {

            System.out.println("VALUE: " + value);

        }

    }

} catch (Exception e) {

    e.printStackTrace();

}

How to tweak the above lambda to get something like this:

KEY: A

VALUE: groupA1

VALUE: groupA2

VALUE: groupA3

KEY: B

VALUE: groupB1

VALUE: groupB2

Map and collect:

Map<String, List<String>> res = lines.stream()

    .map(s -> Arrays.asList(s.split("=")))

    .collect(HashMap::new,

            (map, item) -> map.computeIfAbsent(item.get(0), k -> new ArrayList<>()).add(item.get(1)),

            HashMap::putAll);

Or map and group by:

Map<String, List<String>> res = lines.stream()

        .map(s -> Arrays.asList(s.split("=")))

        .collect(Collectors.groupingBy(s -> s.get(0), Collectors.mapping(v->v.get(1), Collectors.toList())));

1. 
   Stream.collect documentation

Use Collectors.mapping while groupingBy, for more information look at this doc-with-example

Map<String, List<String>> conf = stream.    

   collect(Collectors.groupingBy(s -> s.split("=")[0], Collectors.mapping(v->v.split("=")[1], Collectors.toList())));



    System.out.println(conf); //{A=[groupA1, groupA2, groupA3], B=[groupB1, groupB2]}

If you are open to using a third-party library, the following will work using Eclipse Collections.

ListMultimap<String, String> strings = stream

        .map(s -> s.split("="))

        .collect(Collectors2.toListMultimap(a -> a[0], a -> a[1]));

Collectors2.toListMultimap takes a Function to calculate the key and a separate Function to calculate the value. The ListMultimap<K, V> type is equivalent to Map<K, List<V>>.

Note: I am a committer for Eclipse Collections.

I would rather use apre-compiled Regex Pattern instead of string methods to get the work done. Here's how it looks in practice.

private static final Pattern COMMA_DELIMITER = Pattern.compile("=");



Map<String, List<String>> keyToValuesMap = lines

    .map(l -> COMMA_DELIMITER.splitAsStream(l).toArray(String::new))

    .collect(Collectors.groupingBy(a -> a[0], Collectors.mapping(a -> a[1], 

        Collectors.toList())));

Incidentally use specific exceptions instead of generic Exception. In your case substitute Exception with the specific IOException in your catch block. For a larger data set this approach should definitely outperform others that uses string manipulation.