refactor: Enhance docs, code, add tests in HeronsFormula (#6746)

* refactor: Enhance docs, code, add tests in `HeronsFormula`

* Fix lint

Author: Hardvan

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

@@ -1,33 +1,76 @@
 package com.thealgorithms.maths;
 
 /**
- * Wikipedia for HeronsFormula => https://en.wikipedia.org/wiki/Heron%27s_formula
- * Find the area of a triangle using only side lengths
+ * Heron's Formula implementation for calculating the area of a triangle given
+ * its three side lengths.
+ * <p>
+ * Heron's Formula states that the area of a triangle whose sides have lengths
+ * a, b, and c is:
+ * Area = √(s(s - a)(s - b)(s - c))
+ * where s is the semi-perimeter of the triangle: s = (a + b + c) / 2
+ * </p>
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Heron%27s_formula">Heron's
+ *      Formula - Wikipedia</a>
+ * @author satyabarghav
  */
-
 public final class HeronsFormula {
 
-    /*
-     * A function to get the Area of a Triangle using Heron's Formula
-     * @param s1,s2,s3 => the three sides of the Triangle
-     * @return area using the formula (√(s(s – s1)(s – s2)(s – s3)))
-     * here s is called semi-perimeter and it is the half of the perimeter (i.e; s = (s1+s2+s3)/2)
-     * @author satyabarghav
+    /**
+     * Private constructor to prevent instantiation of this utility class.
      */
     private HeronsFormula() {
     }
 
+    /**
+     * Checks if all three side lengths are positive.
+     *
+     * @param a the length of the first side
+     * @param b the length of the second side
+     * @param c the length of the third side
+     * @return true if all sides are positive, false otherwise
+     */
     private static boolean areAllSidesPositive(final double a, final double b, final double c) {
         return a > 0 && b > 0 && c > 0;
     }
 
+    /**
+     * Checks if the given side lengths satisfy the triangle inequality theorem.
+     * <p>
+     * The triangle inequality theorem states that the sum of any two sides
+     * of a triangle must be greater than the third side.
+     * </p>
+     *
+     * @param a the length of the first side
+     * @param b the length of the second side
+     * @param c the length of the third side
+     * @return true if the sides can form a valid triangle, false otherwise
+     */
     private static boolean canFormTriangle(final double a, final double b, final double c) {
         return a + b > c && b + c > a && c + a > b;
     }
 
+    /**
+     * Calculates the area of a triangle using Heron's Formula.
+     * <p>
+     * Given three side lengths a, b, and c, the area is computed as:
+     * Area = √(s(s - a)(s - b)(s - c))
+     * where s is the semi-perimeter: s = (a + b + c) / 2
+     * </p>
+     *
+     * @param a the length of the first side (must be positive)
+     * @param b the length of the second side (must be positive)
+     * @param c the length of the third side (must be positive)
+     * @return the area of the triangle
+     * @throws IllegalArgumentException if any side length is non-positive or if the
+     *                                  sides cannot form a valid triangle
+     */
     public static double herons(final double a, final double b, final double c) {
-        if (!areAllSidesPositive(a, b, c) || !canFormTriangle(a, b, c)) {
-            throw new IllegalArgumentException("Triangle can't be formed with the given side lengths");
+        if (!areAllSidesPositive(a, b, c)) {
+            throw new IllegalArgumentException("All side lengths must be positive");
+        }
+        if (!canFormTriangle(a, b, c)) {
+            throw new IllegalArgumentException("Triangle cannot be formed with the given side lengths (violates triangle inequality)");
         }
         final double s = (a + b + c) / 2.0;
         return Math.sqrt((s) * (s - a) * (s - b) * (s - c));

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

@@ -3,37 +3,141 @@
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
-public class HeronsFormulaTest {
+/**
+ * Test cases for {@link HeronsFormula}.
+ */
+class HeronsFormulaTest {
+
+    private static final double EPSILON = 1e-10;
+
+    @Test
+    void testRightTriangleThreeFourFive() {
+        Assertions.assertEquals(6.0, HeronsFormula.herons(3, 4, 5), EPSILON);
+    }
+
+    @Test
+    void testTriangleTwentyFourThirtyEighteen() {
+        Assertions.assertEquals(216.0, HeronsFormula.herons(24, 30, 18), EPSILON);
+    }
+
+    @Test
+    void testEquilateralTriangle() {
+        Assertions.assertEquals(0.4330127018922193, HeronsFormula.herons(1, 1, 1), EPSILON);
+    }
+
+    @Test
+    void testScaleneTriangleFourFiveEight() {
+        Assertions.assertEquals(8.181534085976786, HeronsFormula.herons(4, 5, 8), EPSILON);
+    }
+
+    @Test
+    void testEquilateralTriangleLargeSides() {
+        final double side = 10.0;
+        final double expectedArea = Math.sqrt(3) / 4 * side * side;
+        Assertions.assertEquals(expectedArea, HeronsFormula.herons(side, side, side), EPSILON);
+    }
 
     @Test
-    void test1() {
-        Assertions.assertEquals(HeronsFormula.herons(3, 4, 5), 6.0);
+    void testIsoscelesTriangle() {
+        Assertions.assertEquals(12.0, HeronsFormula.herons(5, 5, 6), EPSILON);
     }
 
     @Test
-    void test2() {
-        Assertions.assertEquals(HeronsFormula.herons(24, 30, 18), 216.0);
+    void testSmallTriangle() {
+        Assertions.assertEquals(0.4330127018922193, HeronsFormula.herons(1.0, 1.0, 1.0), EPSILON);
     }
 
     @Test
-    void test3() {
-        Assertions.assertEquals(HeronsFormula.herons(1, 1, 1), 0.4330127018922193);
+    void testLargeTriangle() {
+        Assertions.assertEquals(600.0, HeronsFormula.herons(30, 40, 50), EPSILON);
     }
 
     @Test
-    void test4() {
-        Assertions.assertEquals(HeronsFormula.herons(4, 5, 8), 8.181534085976786);
+    void testDecimalSides() {
+        final double area = HeronsFormula.herons(2.5, 3.5, 4.0);
+        Assertions.assertTrue(area > 0);
+        Assertions.assertEquals(4.330127018922194, area, EPSILON);
     }
 
     @Test
-    public void testCalculateAreaWithInvalidInput() {
+    void testDegenerateTriangleEqualToSumOfOtherTwo() {
         Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(1, 2, 3); });
+    }
+
+    @Test
+    void testDegenerateTriangleVariant2() {
         Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(2, 1, 3); });
+    }
+
+    @Test
+    void testDegenerateTriangleVariant3() {
         Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(3, 2, 1); });
+    }
+
+    @Test
+    void testDegenerateTriangleVariant4() {
         Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(1, 3, 2); });
+    }
 
-        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(1, 1, 0); });
-        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(1, 0, 1); });
+    @Test
+    void testInvalidTriangleSideGreaterThanSum() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(1, 1, 5); });
+    }
+
+    @Test
+    void testZeroFirstSide() {
         Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(0, 1, 1); });
     }
+
+    @Test
+    void testZeroSecondSide() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(1, 0, 1); });
+    }
+
+    @Test
+    void testZeroThirdSide() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(1, 1, 0); });
+    }
+
+    @Test
+    void testNegativeFirstSide() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(-1, 2, 2); });
+    }
+
+    @Test
+    void testNegativeSecondSide() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(2, -1, 2); });
+    }
+
+    @Test
+    void testNegativeThirdSide() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(2, 2, -1); });
+    }
+
+    @Test
+    void testAllNegativeSides() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(-1, -2, -3); });
+    }
+
+    @Test
+    void testAllZeroSides() {
+        Assertions.assertThrows(IllegalArgumentException.class, () -> { HeronsFormula.herons(0, 0, 0); });
+    }
+
+    @Test
+    void testVerySmallTriangle() {
+        final double result = HeronsFormula.herons(0.001, 0.001, 0.001);
+        Assertions.assertTrue(result > 0);
+        Assertions.assertTrue(result < 0.001);
+    }
+
+    @Test
+    void testRightTriangleFiveTwelveThirteen() {
+        Assertions.assertEquals(30.0, HeronsFormula.herons(5, 12, 13), EPSILON);
+    }
+
+    @Test
+    void testRightTriangleEightFifteenSeventeen() {
+        Assertions.assertEquals(60.0, HeronsFormula.herons(8, 15, 17), EPSILON);
+    }
 }