Converting a text file to Map<String, List<String>> using lambda

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.