WORKING

Author: Atri-Sarker

.gitignore

@@ -22,3 +22,6 @@
 # virtual machine crash logs, see http://www.java.com/en/download/help/error_hotspot.xml
 hs_err_pid*
 replay_pid*
+
+# .txt
+*.txt

Factorial.java

@@ -0,0 +1,117 @@
+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Scanner;
+
+/**
+* Program that takes numbers from an input file.
+* For each number, it will evaluate the factorial of the number,
+* using a recursive function.
+* The results will be written onto an output file.
+*
+* @author  Atri Sarker
+* @version 1.0
+* @since   2025-11-17
+*/
+public final class Factorial {
+  /**
+   * Private constructor to satisfy style checker.
+   * @exception IllegalStateException for the utility class.
+   * @see IllegalStateException
+   */
+  private Factorial() {
+    // Prevents illegal states.
+    throw new IllegalStateException("Utility class.");
+  }
+  /**
+   * Recursive function that calculates the factorial of a number.
+   * @param n The number to take the factorial of.
+   * @return The factorial of n.
+   */
+  public static int factorial(final int n) {
+    // BASE CASE [0! = 1, 1! = 1]
+    if (n <= 1) {
+      return 1;
+    } else {
+      // RECURSIVE CALL
+      return n * factorial(n - 1);
+    }
+  }
+
+  /**
+   * Entrypoint of the program.
+   * @param args For command line arguments.
+   */
+  public static void main(final String[] args) {
+    // First argument is the path to the input file.
+    final String inputFilePath = args[0];
+    // Second argument is the path to the output file.
+    final String outputFilePath = args[1];
+    // Print arguments
+    System.out.println("Input file: " + inputFilePath);
+    System.out.println("Output file: " + outputFilePath);
+    try {
+        // Access the input file and create a File object.
+        File inputFile = new File(inputFilePath);
+        // Access the output file and create a File object.
+        File outputFile = new File(outputFilePath);
+        // Scanner that will read the File Object.
+        Scanner fileReader = new Scanner(inputFile);
+        // Create list to store all the ints
+        ArrayList<Integer> listOfInts = new ArrayList<>();
+        // Loop through all available lines
+        while (fileReader.hasNextLine()) {
+          // Add the line to the list
+          // [As an integer]
+          try {
+            // Convert line to an integer
+            int num = Integer.parseInt(fileReader.nextLine());
+            // Add the number to the list
+            listOfInts.add(num);
+          } catch (NumberFormatException error) {
+            // If the line can't be converted to an integer,
+            // the program just ignores the line and continues.
+            continue;
+          }
+        }
+        // Close the file reader
+        fileReader.close();
+        // Convert the list to an array
+        int[] arrOfInts = new int[listOfInts.size()];
+        for (int index = 0; index < listOfInts.size(); index++) {
+          arrOfInts[index] = listOfInts.get(index);
+        }
+        // String to hold all output
+        String output = "";
+        // Go through every number and add the factorial to the output string
+        for (int num : arrOfInts) {
+          // Get the factorial
+          final int result = factorial(num);
+          // Add it to the output
+          output += Integer.toString(result);
+          // Print transformation
+          System.out.print(num);
+          System.out.print("! -> ");
+          System.out.println(result);
+          // Add a newline
+          output += "\n";
+        }
+        // Write output to output file
+        try {
+          // Create a FileWriter object to write to the file
+          FileWriter writer = new FileWriter(outputFile);
+          // Write the output to the file
+          writer.write(output);
+          // Close the writer
+          writer.close();
+        } catch (IOException error) {
+          System.out.println(error);
+        }
+    } catch (IOException error) {
+      System.out.println(error);
+    }
+    // Completion message
+    System.out.println("DONE!");
+  }
+}

NameOfProgram.java

@@ -1,7 +1,7 @@
 /**
-* package-info file for NameOfProgram.
+* package-info file for Factorial.
 *
 * @author  Atri Sarker
 * @version 1.0
-* @since   2025-Month-Day
+* @since   2025-11-17
 */