How to merge lists of Map with Lists values using Java Streams API?

Question

How can I reduce the Map<X, List<String>> grouping by the X.p and join all the list values at the same time, so that I have Map<Integer, List<String>> at the end?

This is what I've tried so far:

class X {
    int p;
    int q;
    public X(int p, int q) { this.p = p; this.q = q; }
}
Map<X, List<String>> x = new HashMap<>();
x.put(new X(123,5), Arrays.asList("A","B"));
x.put(new X(123,6), Arrays.asList("C","D"));
x.put(new X(124,7), Arrays.asList("E","F"));
Map<Integer, List<String>> z = x.entrySet().stream().collect(Collectors.groupingBy(
    entry -> entry.getKey().p, 
    mapping(Map.Entry::getValue, 
        reducing(new ArrayList<>(), (a, b) -> { a.addAll(b); return a; }))));
System.out.println("z="+z);

But the result is: z={123=[E, F, A, B, C, D], 124=[E, F, A, B, C, D]}.

I want to have z={123=[A, B, C, D], 124=[E, F]}

Answer 1

Here's one way to do it using two Stream pipelines :

Map<Integer, List<String>> z = 
// first process the entries of the original Map and produce a 
// Map<Integer,List<List<String>>>
    x.entrySet()
     .stream()
     .collect(Collectors.groupingBy(entry -> entry.getKey().p, 
                                    mapping(Map.Entry::getValue,
                                            toList())))
// then process the entries of the intermediate Map and produce a 
// Map<Integer,List<String>>
     .entrySet()
     .stream()
     .collect (toMap (Map.Entry::getKey,
                      e -> e.getValue()
                            .stream()
                            .flatMap(List::stream)
                            .collect(toList())));

Java 9 is supposed to add a flatMapping Collector, that would make your life easier (I learned about this new feature thanks to Holger).

Output :

z={123=[A, B, C, D], 124=[E, F]}