展平地图 <Integer, Map<String, Boolean> &gt;到列表 <Boolean>

Question

Using java 8 streams, how do I flatten a Map<Integer, Map<String, Boolean>> to just a List<Boolean> so that the final list will contain all the booleans across all values?

Here is an example of the data:

1 -> [{"ABC" -> true}, {"DEF" -> true}]
2 -> [{"DEF" -> false}, {"EFG" -> true}]
3 -> [{"ABC" -> true}, {"DEF" -> false}]

I want to flatten this to a List<Boolean> so that my list will contain:

{true, true, false, true, true, false}

Order is not important. Content is important.

Answer 1

Try this:

        Map<Integer, Map<String, Boolean>> map = new HashMap<Integer, Map<String, Boolean>>();
        final List<Boolean> collect = map
            .values()
            .stream()
            .flatMap(it -> it.values().stream())
            .collect(Collectors.toList());

Answer 2

You can do something like:

  final Map<Integer, List<Entry>> map = new HashMap<>();
  map.put(1, Arrays.asList(new Entry("ABC", true), new Entry("DEF", true)));
  map.put(2, Arrays.asList(new Entry("DEF", false), new Entry("EFG", true)));
  map.put(3, Arrays.asList(new Entry("ABC", true), new Entry("DEF", false)));

  final List<Boolean> booleans = map.entrySet()
      .stream()
      .flatMap(entry -> entry.getValue().stream())
      .map(entry -> entry.value)
      .collect(Collectors.toList());

  System.out.println(booleans);

But be aware that the order will not be saved here as this is a HashMap .

Answer 3

have a look at the following:

final List<Boolean> collect = map.values().stream() // Stream<Map<String,Boolean>>
    .map(Map::values)                               // Stream<Collection<Boolean>>
    .flatMap(Collection::stream)                    // Stream<Boolean>
    .collect(Collectors.toList());    

The difference from this and the other answers provided is, that every line only contains one operation (which I personally prefer, its more readable).

Answer 4

Based on

I want to flatten this to a List so that my list will contain: {true, true, false, true, true, false}

the solution could be like this

Map<Integer, Map<String, Boolean>> map = new HashMap<>();
    map.put(1, new HashMap<String, Boolean>(){{put("ABC", true);put("DEF", true);}});
    map.put(2, new HashMap<String, Boolean>(){{put("DEF", false);put("EFG", true);}});
    map.put(3, new HashMap<String, Boolean>(){{put("ABC", true);put("DEF", false);}});

    System.out.println(map.entrySet().stream().flatMap( t -> t.getValue().values().stream()).collect(Collectors.toList()));