Refactor factorial, add unit tests (#4266)

Author: albina-astr

src/main/java/com/thealgorithms/maths/Factorial.java

@@ -1,15 +1,6 @@
 package com.thealgorithms.maths;
 
 public class Factorial {
-
-    /* Driver Code */
-    public static void main(String[] args) {
-        assert factorial(0) == 1;
-        assert factorial(1) == 1;
-        assert factorial(5) == 120;
-        assert factorial(10) == 3628800;
-    }
-
     /**
      * Calculate factorial N using iteration
      *
@@ -18,11 +9,12 @@ public static void main(String[] args) {
      */
     public static long factorial(int n) {
         if (n < 0) {
-            throw new IllegalArgumentException("number is negative");
+            throw new IllegalArgumentException("Input number cannot be negative");
         }
         long factorial = 1;
-        for (int i = 1; i <= n; factorial *= i, ++i)
-            ;
+        for (int i = 1; i <= n; ++i) {
+            factorial *= i;
+        }
         return factorial;
     }
 }

src/test/java/com/thealgorithms/maths/FactorialTest.java

@@ -5,9 +5,19 @@
 import org.junit.jupiter.api.Test;
 
 public class FactorialTest {
+    private static final String EXCEPTION_MESSAGE = "Input number cannot be negative";
 
     @Test
-    public void test() {
+    public void testWhenInvalidInoutProvidedShouldThrowException() {
+        IllegalArgumentException exception = assertThrows(IllegalArgumentException.class, () -> Factorial.factorial(-1));
+        assertEquals(exception.getMessage(), EXCEPTION_MESSAGE);
+    }
+
+    @Test
+    public void testCorrectFactorialCalculation() {
+        assertEquals(1, Factorial.factorial(0));
+        assertEquals(1, Factorial.factorial(1));
         assertEquals(120, Factorial.factorial(5));
+        assertEquals(3628800, Factorial.factorial(10));
     }
 }