Test

app/src/main/java/control/Single.java

@@ -42,9 +42,15 @@ public static int maxArray(int[] arr) {
    * This method calculates the sum of the first n natural numbers, modulo m.
    *
    * @param n The number of natural numbers to sum.
-   * @param m The modulus.
+   * @param m The non-zero modulus.
+   * @throws IllegalArgumentException if m is zero.
    */
   public static int sumModulus(int n, int m) {
+    // Validate first so modulo by zero produces a clearer exception than % would.
+    if (m == 0) {
+      throw new IllegalArgumentException("Modulus must be non-zero");
+    }
+
     Vector<Integer> multiples = new Vector<Integer>();
     for (int i = 0; i < n; i++) {
       if (i % m == 0) {

app/src/test/java/control/SingleTest.java

@@ -1,6 +1,7 @@
 package control;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
 
@@ -33,5 +34,6 @@ public void testSumModulus() {
     assertEquals(20, Single.sumModulus(10, 2));
     assertEquals(18, Single.sumModulus(10, 3));
     assertEquals(12, Single.sumModulus(10, 4));
+    assertThrows(IllegalArgumentException.class, () -> Single.sumModulus(10, 0));
   }
 }