Dynamic grouping by specific attributes with Collection.stream

Question

I am trying to group a list of objects by mulitple attributes, by using Java 8 Collection-Stream.

This works pretty well:

public class MyClass
{
   p         


        

Answer 1

Instead of list of names, you could also consider supplying a list of functions (with one mandatory) to group your elements.

These functions should map an element of MyClass to an object, so you can use Function.

private static Map, List> groupListBy(List data, Function mandatory, Function... others) {
   return data.stream()
              .collect(groupingBy(cl -> Stream.concat(Stream.of(mandatory), Stream.of(others)).map(f -> f.apply(cl)).collect(toList())));
}

And some example of calls:

groupListBy(data, m -> m.type); //group only by type
groupListBy(data, m -> m.type, m -> m.module); //group by type and by module

Of course you can make this method generic so that it returns a Map, List> with functions of the type U -> Object.