Java - Appending Arraylist to an already existing ArrayList [duplicate]

Question

So here are my declarations:

ArrayList<ArrayList<ArrayList<String>>> level1 = new ArrayList<>();
ArrayList<ArrayList<String>> level2 = new ArrayList<>();
ArrayList<String> level3 = new ArrayList<>();

So I first run through and add one level3 item to each level2 item, and then add the level2 items to level1. End result will look like this:

level1(0)
  level2(0)
   level3(0) Tall
level1(1)
   level2(0)
     level3(0) Short
level1(2)
   level2(0)
     level3(0) Thin

Now, what do I do if I want to add an additional level2? So I would take the data above and turn it into this:

level1(0)
  level2(0)
    level3(0) Tall
  level2(1)
    level3(0) Rich
level1(1)
   level2(0)
     level3(0) Short
  level2(1)
    level3(0) Nice
level1(2)
   level2(0)
     level3(0) Thin
  level2(1)
    level3(0) Mean

Doing level1.add(0, level2) doesn't work, it just pushes everything down and now level1 has 4 elements instead of 3. What am I missing?

That's to add an ArrayList to another ArrayList, I need to edit\append an ArrayList that is within an ArrayList. — Marcel Marino
– Marcel Marino, Commented Sep 9, 2016 at 13:12

puhlen · Accepted Answer · 2016-09-08 23:36:25Z

1

The problem is you don't want to add to level1, you are trying to add to level2. First you need to get the level2 out, then you can add to it.

level1.get(0).add(level3)

answered Sep 8, 2016 at 23:36

puhlen

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1 Comment

Marcel Marino Over a year ago

Oh, I see. I was doing level1.get(0).add(level2) and that was getting blocked.

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