refactor: Enhance docs, code, add tests in HarshadNumber (#6745)

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

@@ -1,25 +1,135 @@
 package com.thealgorithms.maths;
 
-import static org.junit.jupiter.api.Assertions.assertFalse;
-import static org.junit.jupiter.api.Assertions.assertTrue;
-
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
-public class HarshadNumberTest {
+/**
+ * Test class for {@link HarshadNumber}.
+ * Tests various scenarios including positive cases, edge cases, and exception
+ * handling.
+ */
+class HarshadNumberTest {
 
     @Test
-    public void harshadNumber() {
+    void testValidHarshadNumbers() {
+        // Single digit Harshad numbers (all single digits except 0 are Harshad numbers)
+        Assertions.assertTrue(HarshadNumber.isHarshad(1));
+        Assertions.assertTrue(HarshadNumber.isHarshad(2));
+        Assertions.assertTrue(HarshadNumber.isHarshad(3));
+        Assertions.assertTrue(HarshadNumber.isHarshad(4));
+        Assertions.assertTrue(HarshadNumber.isHarshad(5));
+        Assertions.assertTrue(HarshadNumber.isHarshad(6));
+        Assertions.assertTrue(HarshadNumber.isHarshad(7));
+        Assertions.assertTrue(HarshadNumber.isHarshad(8));
+        Assertions.assertTrue(HarshadNumber.isHarshad(9));
 
-        assertTrue(HarshadNumber.isHarshad(18));
-        assertFalse(HarshadNumber.isHarshad(-18));
-        assertFalse(HarshadNumber.isHarshad(19));
-        assertTrue(HarshadNumber.isHarshad(999999999));
-        assertFalse(HarshadNumber.isHarshad(0));
+        // Two digit Harshad numbers
+        Assertions.assertTrue(HarshadNumber.isHarshad(10)); // 10 / (1 + 0) = 10
+        Assertions.assertTrue(HarshadNumber.isHarshad(12)); // 12 / (1 + 2) = 4
+        Assertions.assertTrue(HarshadNumber.isHarshad(18)); // 18 / (1 + 8) = 2
+        Assertions.assertTrue(HarshadNumber.isHarshad(20)); // 20 / (2 + 0) = 10
+        Assertions.assertTrue(HarshadNumber.isHarshad(21)); // 21 / (2 + 1) = 7
 
-        assertTrue(HarshadNumber.isHarshad("18"));
-        assertFalse(HarshadNumber.isHarshad("-18"));
-        assertFalse(HarshadNumber.isHarshad("19"));
-        assertTrue(HarshadNumber.isHarshad("999999999"));
-        assertTrue(HarshadNumber.isHarshad("99999999999100"));
+        // Three digit Harshad numbers
+        Assertions.assertTrue(HarshadNumber.isHarshad(100)); // 100 / (1 + 0 + 0) = 100
+        Assertions.assertTrue(HarshadNumber.isHarshad(102)); // 102 / (1 + 0 + 2) = 34
+        Assertions.assertTrue(HarshadNumber.isHarshad(108)); // 108 / (1 + 0 + 8) = 12
+
+        // Large Harshad numbers
+        Assertions.assertTrue(HarshadNumber.isHarshad(1000)); // 1000 / (1 + 0 + 0 + 0) = 1000
+        Assertions.assertTrue(HarshadNumber.isHarshad(1002)); // 1002 / (1 + 0 + 0 + 2) = 334
+        Assertions.assertTrue(HarshadNumber.isHarshad(999999999)); // 999999999 / (9*9) = 12345679
+    }
+
+    @Test
+    void testInvalidHarshadNumbers() {
+        // Numbers that are not Harshad numbers
+        Assertions.assertFalse(HarshadNumber.isHarshad(11)); // 11 / (1 + 1) = 5.5
+        Assertions.assertFalse(HarshadNumber.isHarshad(13)); // 13 / (1 + 3) = 3.25
+        Assertions.assertFalse(HarshadNumber.isHarshad(17)); // 17 / (1 + 7) = 2.125
+        Assertions.assertFalse(HarshadNumber.isHarshad(19)); // 19 / (1 + 9) = 1.9
+        Assertions.assertFalse(HarshadNumber.isHarshad(23)); // 23 / (2 + 3) = 4.6
+        Assertions.assertFalse(HarshadNumber.isHarshad(101)); // 101 / (1 + 0 + 1) = 50.5
+    }
+
+    @Test
+    void testZeroThrowsException() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad(0));
+    }
+
+    @Test
+    void testNegativeNumbersThrowException() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad(-1));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad(-18));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad(-100));
+    }
+
+    @Test
+    void testValidHarshadNumbersWithString() {
+        // Single digit Harshad numbers
+        Assertions.assertTrue(HarshadNumber.isHarshad("1"));
+        Assertions.assertTrue(HarshadNumber.isHarshad("2"));
+        Assertions.assertTrue(HarshadNumber.isHarshad("9"));
+
+        // Two digit Harshad numbers
+        Assertions.assertTrue(HarshadNumber.isHarshad("10"));
+        Assertions.assertTrue(HarshadNumber.isHarshad("12"));
+        Assertions.assertTrue(HarshadNumber.isHarshad("18"));
+
+        // Large Harshad numbers
+        Assertions.assertTrue(HarshadNumber.isHarshad("1000"));
+        Assertions.assertTrue(HarshadNumber.isHarshad("999999999"));
+        Assertions.assertTrue(HarshadNumber.isHarshad("99999999999100"));
+    }
+
+    @Test
+    void testInvalidHarshadNumbersWithString() {
+        // Numbers that are not Harshad numbers
+        Assertions.assertFalse(HarshadNumber.isHarshad("11"));
+        Assertions.assertFalse(HarshadNumber.isHarshad("13"));
+        Assertions.assertFalse(HarshadNumber.isHarshad("19"));
+        Assertions.assertFalse(HarshadNumber.isHarshad("23"));
+    }
+
+    @Test
+    void testStringWithZeroThrowsException() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad("0"));
+    }
+
+    @Test
+    void testStringWithNegativeNumbersThrowsException() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad("-1"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad("-18"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad("-100"));
+    }
+
+    @Test
+    void testNullStringThrowsException() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad(null));
+    }
+
+    @Test
+    void testEmptyStringThrowsException() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad(""));
+    }
+
+    @Test
+    void testInvalidStringThrowsException() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad("abc"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad("12.5"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad("12a"));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> HarshadNumber.isHarshad(" 12 "));
+    }
+
+    @Test
+    void testMaxLongValue() {
+        // Test with a large number close to Long.MAX_VALUE
+        long largeHarshadCandidate = 9223372036854775800L;
+        // This specific number may or may not be Harshad, just testing it doesn't crash
+        try {
+            HarshadNumber.isHarshad(largeHarshadCandidate);
+        } catch (Exception e) {
+            Assertions.fail("Should not throw exception for valid large numbers");
+        }
     }
 }