Arraylist inside arraylist, how do I clear the last

I use while loop to read a text file and create new arraylists out of portions of the text file and then place those arraylists inside another arraylist. My problem is that I don't know how to add my temporary arraylist<String> parse into the mainAList and then clear it for the next loop without affecting all the parse arraylists inside the arraylist<arraylist<string>> mainAList. More info inside the comments below in the code.

This is a sample code, it is from larger file. It is shortened but has all the moving parts

ArrayList<ArrayList<String>> mainAList = new ArrayList<ArrayList<String>>();
ArrayList<String> parse = new ArrayList<>();

try (Scanner filereader = new Scanner(Paths.get("name_of_file.txt"))) {
    while (filereader.hasNextLine()) {
        parse.add(filereader.nextLine());
        if (parse.get(parse.size()-1).length() == 0 || !filereader.hasNextLine()) {
            // if the file reader detects empty line or has no more lines
            //  the arraylist called parse gets added to the mainAList
            mainAList.add(parse);
        }
        parse.clear();
        // here is my problem. How do I clear parse so for the next patch of lines
         //  so I don't add the same things + more into the next slot of mainAList
         // every time the loop runs?
         // I understand my issue is not how to correctly clear the parse but 
         // how to add the arraylists correctly into the mainAList so I can alter them individually
   }
}

The file that is read is like this:
a
b
c
d

f
g

h
i
j
k
l

etc..

There are two problems with your current approach:

1. parse will use the same reference for all inner ArrayLists, so when you call parse.clear() it will empty all inner lists of your mainAList. This can be solved by creating a copy of the parse list when you add it (i.e. mainAList.add(new ArrayList<>(parse));)
2. You want to clear the list inside the if as well, after you've added the copy to your mainAList.

In addition, I assume you don't want the empty lines added to the individual inner Lists, so you could read the line first, and either add the entire list to mainAList and clear parse inside the if-statement, or add the line to the parse-list.

A slightly better alternative, is to just create a new list instead, so you won't need the clear and creating a copy of the list.

In total:

ArrayList<ArrayList<String>> mainAList = new ArrayList<ArrayList<String>>();
ArrayList<String> parse = new ArrayList<>();

try (Scanner filereader = new Scanner(System.in)) {
  while (filereader.hasNextLine()) {
    String line = filereader.nextLine();
    if (line.length() == 0 || !filereader.hasNextLine()) {
        // if the file reader detects empty line or has no more lines
        // the arraylist called parse gets added to the mainAList
        mainAList.add(parse);
        // and then a new parse-list is created for the next group of inputs:
        parse = new ArrayList<>();
    } else{
        parse.add(line);
    }
  }
}

Try it online.

Try this instead of clearing the list everytime.

ArrayList<ArrayList<String>> mainAList = new ArrayList<ArrayList<String>>();

try (Scanner filereader = new Scanner(Paths.get("name_of_file.txt"))) {
    while (filereader.hasNextLine()) {
    	ArrayList<String> parse = new ArrayList<>();
        parse.add(filereader.nextLine());
        if (parse.get(parse.size()-1).length() == 0 || !filereader.hasNextLine()) {
         mainAList.add(parse);
        }
   }
}

Your problem is that your mainAList will be empty in the end because objects are reference types. In order to fix that problem, you have to add a copy of parse to the mainAList and not the parse arraylist itself

replace this line

mainAList.add(parse);

with this

mainAList.add(new ArrayList<>(parse));