several arrayList to single List

I’m working on an app that deals with animals by category.

  public class Animal { 
  private long id; 
  private color String ;
  ... 
   }

I have several arrayList with animal objects.
I manage to gather all my arrayList in another arrayList

So i have:

ArrayList <ArrayList<Animal>> 

my goal is to put all arrayList in a single arrayList

In summary:

   Before :  ArrayList <ArrayList<Animal>>     
   After:  ArrayList<Animal> with a same list of Animals

I manage to do it in procedural programming easily but I don’t know how to do it in functional programming with Stream from Java 8

>Solution :

You can solve with Flatmap with stream, Collect

ArrayList<ArrayList<Animal>> SingleListofAnimal = new  ArrayList<ArrayList<Animal>>();

 SingleListofAnimal  = List.stream()
                        .flatMap(List::stream)
                        .collect(Collectors.toList());

Let me know if it works for you