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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 a pre-compiled Regex Pattern instead of string methods to get the work done. Here's how it looks in practice.

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



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

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

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

        Collectors.toList())));

For a larger data set this approach should definitely outperform others that uses string manipulation.

Incidentally use specific exceptions instead of generic Exception. In your case substitute Exception with the specific IOException in your catch block.