Java 8 Streams - Grouping into Single value [duplicate]

Question

I am currently working with a List<Map<String, Object>> where I am trying to group across various keys within the map. This seems to work nicely using the Java 8 Streams:

Map<Object, Map<Object, List<Map<String, Object>>>> collect =
   list
   .stream()
   .collect(Collectors.groupingBy(
       item -> item.get("key1"),
       Collectors.groupingBy(item -> item.get("key2"))
   ));

As expected this give me a Map<Object, Map<Object, List<Map<String, Object>>>> which works well where the possible grouped results are greater than 1.

I have various examples where the grouping being done will always result in a single item in the lowest level list, for example.

List of Rows

{
  [reference="PersonX", firstname="Person", dob="test", lastname="x"],
  [reference="JohnBartlett", firstname="John", dob="test", lastname="Bartlett"]
}

Grouped by reference

Currently - grouped into a list with 1 Map<String,Object>

[PersonX, { [reference="PersonX", firstname="Person", dob="test", lastname="x"]}],
[JohnBartlett, { [reference="JohnBartlett", firstname="John", dob="test", lastname="Bartlett"]}]

Preference - No List just a single Map<String,Object>

[PersonX, [reference="PersonX", firstname="Person", dob="test", lastname="x"]],
[JohnBartlett, [reference="JohnBartlett", firstname="John", dob="test", lastname="Bartlett"]]

is there a way within the streams to force the output for these instances to be Map<Object, Map<Object, Map<String, Object>>> - so a single Map<String,Object> rather than a List of them?

Any help would be greatly appreciated.

Answer 1

If I understood correctly, then for the cases where you are sure that there is a single item, you should just replace:

 .collect(Collectors.groupingBy(
   item -> item.get("key1"),
   Collectors.toMap(item -> item.get("key2"), Function.identity())
 ));

You can even provide a third argument as a BinaryOperator to merge your same entries (in case you need to)