GroupBy and merge into a single list in java

Question

Java 8 provides a groupingBy function in Collectors, but it gives a map of elements. I need to convert the map into a single list.

Also it returns the result, but instead what I want is to perform the grouping operation to the list itself so that I do not have to reassign it, as reassigning is not possible in a lambda expression.

How can I achieve this?

I want to achieve this:

Map<String, List<Node>> nodeListGrouped = nodeList.stream().collect(Collectors.groupingBy(node -> node.getGroupName()));

convert nodeListGrouped into a single List

But nodeListGrouped.values() returns Collection(List)

Answer 1

I think this is what you are looking for (I have not compiled this though )

List<Node> list = 
         nodeList.stream()
                 .collect(Collectors.collectingAndThen(
                     Collectors.groupingBy(Node::getGroupName),
                     map -> map.values()
                               .stream()
                               .flatMap(List::stream)
                               .collect(Collectors.toList())
));