Load an ArrayList with nested ArrayLists from a file using Java

I have an arraylist like ArrayList<ArrayList<String>>. I have saved my main ArrayList in a text file by using the following code.

 ArrayList<ArrayList<String>> ar2 = new ArrayList<>();
 FileWriter writer = new FileWriter("path/to/the/text/file");
        for(ArrayList<String> str: ar2) {
            writer.write(str + System.lineSeparator());

        }
        writer.close();

Now, I want to load the saved data from the file to the same ArrayList ar2 in every new run. I tried several methods but the nested arraylists are loaded as Strings. How can I overcome from this issue?

Answer

Read the file contents, split by line separator and then remove the brackets and split again to get the list items. Something like

File file = new File("path/to/the/text/file");
FileReader reader = new FileReader(file);

char[] chars = new char[(int) file.length()];
reader.read(chars);

String fileContent = new String(chars);
ArrayList<ArrayList<String>> readList = new ArrayList<>();

String[] splittedArr = fileContent.split(System.lineSeparator());
for(String list : splittedArr) {
    list = list.replace("[", "").replace("]", ""); //removing brackets as the file will have list as [a, b, c]
    List<String> asList = Arrays.asList(list.split(", "));//splitting the elements by comma and space
    readList.add(new ArrayList<>(asList));
}
reader.close();