My Recursion program I made Should work , I want to print to console before adding the File Writer part but maybe I am missing something simple. I would Appreciate any help I can get, I am new here, so forgive any Formatting errors I made, feedback is welcome. Thank you.

• Explanation:

For Each Letter in the Mnemonics Array Assigned to an Input number like:”623″ the program will recursively call itself and combine different letters return, a combo, and add it back to the ‘prefix’ letter it was called for.

        import java.util.*;
        import java.io.*;

        /**
         * Uses recursion to make Mnemoics.
         *
         * @author K.Dot
         * @version 1.0
         * @since 2024-05-06
         */

        // MnemonicsCombos class
        public final class Factorial {

          /** Private constructor to prevent instantiation. 
           * @return */
          private void MnemonicsCombos() {
            throw new UnsupportedOperationException("Cannot instantiate");
          }

          public static void main(final String[] args) {
            final File inputFile = new File("input.txt");
            final String outputFile = "output.txt";
            ArrayList<String> MnemonicsOutputList = new ArrayList();
              /*
              * - This is where the dictionary, for Mnemonics goes.
              * - This is where the call for Mnemonics go.
              */
              Map<String, String> lettersForDigitDictionary = new HashMap<>();
              lettersForDigitDictionary.put("1", "1");
              lettersForDigitDictionary.put("2", "ABC");
              lettersForDigitDictionary.put("3", "DEF");
              lettersForDigitDictionary.put("4", "GHI");
              lettersForDigitDictionary.put("5", "JKL");
              lettersForDigitDictionary.put("6", "MNO");
              lettersForDigitDictionary.put("7", "PQRS");
              lettersForDigitDictionary.put("8", "TUV");
              lettersForDigitDictionary.put("9", "WXYZ");
              lettersForDigitDictionary.put("0", " ");
            
            try {
              Scanner sc = new Scanner(inputFile);
              FileWriter fileWriter = new FileWriter(outputFile);
              BufferedWriter writer = new BufferedWriter(fileWriter);
              while(sc.hasNextLine()){
                int ErrorlineIter = 1;
                try{
                  String mnemonicNumAsStr = sc.nextLine();
                  MnemonicsOutputList = ListAllMnemonics(mnemonicNumAsStr, MnemonicsOutputList, lettersForDigitDictionary);
                  String[] MnemonicsOutputArray = MnemonicsOutputList.toArray(new String[0]);
                  for (String combo : MnemonicsOutputArray) {
                    System.out.print(combo + " ");
                }
                }catch(Exception e){
                  System.out.println("Error at line: " + ErrorlineIter);
                }
                ErrorlineIter++;
              }
              System.out.println("Done.");
              writer.close();
              sc.close();
            } catch (Exception e) {
              System.out.println("Invalid input path!");
            }
          }

          /**
           * Recursive method that gets the MnemonicsCombos of a number.
           *
           * @param someNumString is the 3 letter umber we send the console.
           * @param MnemonicsCombos is the combination we get back.
           * @param AlphaNumMap is the dictionary that stores the map
           */
          private static ArrayList ListAllMnemonics(String someNumString, ArrayList<String> MnemonicsCombos, Map<String, String> alphNumMap) {
            if (someNumString.length() == 0){
              return MnemonicsCombos;
            }
            else {
              // We do the recursive Call case.
              char currentNumFromString = someNumString.charAt(0);
              String dictDefinition = alphNumMap.get(currentNumFromString);
              //For each letter in the dict-def. Which will be known as prefix. (See next line)
              for (char frontLetter : dictDefinition.toCharArray()) {
                //Call funtion to get suffix cominations and assign them to to each prefix letter.
                // We Assign it to a temporary list called "listOfSuffixes".
                List<String> listOfSuffixes = ListAllMnemonics(someNumString.substring(1), MnemonicsCombos, alphNumMap);
                for (String suffixCombination : listOfSuffixes) {
                  // The (prefix + suffix) = "newValue" I will assign to Mnemonics to return it.
                  String newValue = frontLetter + suffixCombination;
                  MnemonicsCombos.add(newValue);
                }
              }
              return MnemonicsCombos;
            }
          }
        }

I can’t say I tried much, I am utterly stumped as to why it is crashing, chat GPT could not be of any help.