How iterate an ArrayList inside various others ArrayLists?

Question

I was notified that is possible to create "infinite" ArrayLists inside others, such as in the code below:

import java.util.ArrayList;

public class Main
{
    public static void main(String[] args) {
        ArrayList<ArrayList<ArrayList<ArrayList<ArrayList<Object>>>>> List = new ArrayList<ArrayList<ArrayList<ArrayList<ArrayList<Object>>>>>();
    }
}

And I want know about how to iterate them (with foreach or other loop types)

Java's type system doesn't like it when you do stuff like this. In any case, the only practical way to iterate through arbitrarily-deeply-nested lists is with recursion — Green Cloak Guy, Aug 17 '21 at 17:07
If you have a data structure like this, you are doing a lot of things wrong. Time to rethink some dicisions. Apart from that a list of lists is just a list. So you can iterate it like any other list. — wannaBeDev, Aug 17 '21 at 17:10
Really it's doing the things much more difficult that should be — UserOfStackOverFlow, Aug 17 '21 at 17:22

score 3 · Accepted Answer · answered Aug 17 '21 at 17:07

3

You could try to traverse it like a composite:

public static void traverse(ArrayList<T> arg) {
   arg.forEach(
    (n) -> if (n instanceof ArrayList) { 
        traverse(n)
      } else {
        doStomething(n)
      }
  );
}

answered Aug 17 '21 at 17:07

Vulwsztyn

2,140
1
12
20

1

Thanks! I'll try to understand it and use for decode the loop. – UserOfStackOverFlow Aug 19 '21 at 13:51
I've searching how do this is other languages, and you answer is very similar of this: https://stackoverflow.com/questions/7106410/looping-through-arrays-of-arrays – UserOfStackOverFlow Sep 13 '21 at 16:13
yep, it's the same concept – Vulwsztyn Sep 13 '21 at 16:22

score 2 · Answer 2 · answered Aug 17 '21 at 17:14

2

IMHO, if someone writes such structures - it's some kind of a maniac. But still, you can follow this kind of pattern in order to traverse:

@Test
    public void test4() {
        List<String> list1 = new ArrayList<>();
        list1.add("abc");
        list1.add("def");

        List<List<String>> list2 = new ArrayList<>();
        list2.add(list1);

        list2.stream().flatMap(Collection::stream).forEach(System.out::println);
    }
```

answered Aug 17 '21 at 17:14

Andriy Zhuk

133
6

Interesting answer, seems more simple than the Vulwsztyn solution, although his answer like more robust, I not understand well. – UserOfStackOverFlow Aug 18 '21 at 13:29
What this component does :: ? – UserOfStackOverFlow Sep 06 '21 at 19:33
@UserOfStackOverFlow this is a short way to reference an existing method within lambda. E.g.: 'flatMap(Collection::stream)' can be replaced with 'flatMap(param -> param.stream())', which would make it look longer without any neccessity. You can find more detailed info here: https://www.baeldung.com/java-method-references – Andriy Zhuk Sep 07 '21 at 20:26

How iterate an ArrayList inside various others ArrayLists?

2 Answers2