how to store ArrayList Values if programe crashes?

I am running a 4*4 maze all possible solution finding programme.I have set up a method that takes a random combination Value of"R, G, B, Y" as aStrin and checks if the given String is a valid solution or not. for instance, the given String is "RGGGBYYBBGYBGRR".

new Thread(() -> {       //List<String> temp1 = new ArrayList<>();       String[] letters = {"R","G","B","Y"};       int done = 0;       Cell start = MazeBuilder.mazeFor(REPO_NAME);       while (done != 1) {           String str = "";           int r = ThreadLocalRandom.current().nextInt(10, 48);           //done = check(temp1, letters, done, start, str, r);           for (int j = 0; j < r; j++) {               Random rnd = new Random();               int index = rnd.nextInt(letters.length);               str += letters[index];           }               if (Helper.isValidSolution(start, str)) {                  done = 1;                   try {                       FileWriter FW = new FileWriter("ansGrid.txt");                       FW.write(str);                       FW.close();                   } catch (IOException e) {                       e.printStackTrace();                   }                }       }   }).start();

so my question is... if the programme crashes or it runs out of memory, how can I store the values of the temp1 variable in a file?