How to Convert a Map<String, List<List<String>>> to Map<String, List<String>> in java 8 functional APIs

I have a class like this;

class: {
   id: "",
   sub: [{
       type: 1,
       value: ""
   }]
}

Now, I want group class first by id;

class.stream().collect(Collectors.groupingBy(class::id))

Then I want flat sub-value to a List<String>, I have the following:

class.stream().collect(Collectors.groupingBy(class::id,
  Collectors.mapping(e ->
    e.sub().stream()
      .filter(v -> v.type().equals(1))
       .map(sub::value),
    Collectors.toList()
  )))

It returns a Map<String, List<List<String>>>

So, how can I convert a Map<String, List<List<String>>> to Map<String, List<String>> using the Java 8 functional API?

You can use the Map collector to group by keys and merge the values.

Example (Java 14 & above):

class Test {

	// sample data types
	record Sub ( int type, String value ){}
    record Clazz (String id, List<Sub> sub){}

	public static void main(String[] args)
	{
		// sample data
		List<Clazz> classes = List.of(
			new Clazz("id1",List.of(new Sub(1,"A"),new Sub(2,"B"))),
			new Clazz("id2",List.of(new Sub(3,"C"),new Sub(4,"D"))),
			new Clazz("id1",List.of(new Sub(5,"E"),new Sub(6,"A")))
		);

		Map<String, List<String>> map = classes.stream()
			.collect(Collectors.toMap(
				// key for new map: c.id
				c -> c.id,
				
				// value for new map: c.sub[].value as a list
				c -> c.sub.stream().map(s -> s.value).toList(),
				
				// duplicate value merging (grouping) : merge two value lists, no duplicates
				(v1, v2) -> Stream.concat(v1.stream(), v2.stream()).distinct().toList()
			));

		// print resulting data
		map.forEach((k,v) -> System.out.println(k + "\t->\t" + String.join(",", v)));
	}
}

Output:

id2	->	C,D
id1	->	A,B,E

Just like @Sametcey suggested you can use flatMap.<br> Whenever you encounter something that uses nested lists (just like your Map<String, List<List>>, but it's possible to use it on deeper level like Map<String, List<List<List>>>) flatMap will help you with that.

It will transform your stream into a single flattened stream. In other words: one list that you can access and manipulate.