refactor: Enhance docs, remove main. add more tests in `HexaDecimal…

…ToDecimal`

src/main/java/com/thealgorithms/conversions/HexaDecimalToDecimal.java

@@ -1,39 +1,45 @@
 package com.thealgorithms.conversions;
 
-import java.util.Scanner;
-
+/**
+ * Utility class for converting a hexadecimal string to its decimal representation.
+ * <p>
+ * A hexadecimal number uses the base-16 numeral system, with the following characters:
+ * <ul>
+ *   <li>Digits: 0-9</li>
+ *   <li>Letters: A-F (case-insensitive)</li>
+ * </ul>
+ * Each character represents a power of 16. For example:
+ * <pre>
+ *   Hexadecimal "A1" = 10*16^1 + 1*16^0 = 161 (decimal)
+ * </pre>
+ *
+ * <p>This class provides a method to perform the conversion without using built-in Java utilities.</p>
+ */
 public final class HexaDecimalToDecimal {
     private HexaDecimalToDecimal() {
     }
 
-    // convert hexadecimal to decimal
+    /**
+     * Converts a hexadecimal string to its decimal integer equivalent.
+     * <p>The input string is case-insensitive, and must contain valid hexadecimal characters [0-9, A-F].</p>
+     *
+     * @param hex the hexadecimal string to convert
+     * @return the decimal integer representation of the input hexadecimal string
+     * @throws IllegalArgumentException if the input string contains invalid characters
+     */
     public static int getHexaToDec(String hex) {
         String digits = "0123456789ABCDEF";
         hex = hex.toUpperCase();
         int val = 0;
+
         for (int i = 0; i < hex.length(); i++) {
             int d = digits.indexOf(hex.charAt(i));
+            if (d == -1) {
+                throw new IllegalArgumentException("Invalid hexadecimal character: " + hex.charAt(i));
+            }
             val = 16 * val + d;
         }
-        return val;
-    }
 
-    // Main method gets the hexadecimal input from user and converts it into Decimal output.
-    public static void main(String[] args) {
-        String hexaInput;
-        int decOutput;
-        Scanner scan = new Scanner(System.in);
-
-        System.out.print("Enter Hexadecimal Number : ");
-        hexaInput = scan.nextLine();
-
-        // convert hexadecimal to decimal
-        decOutput = getHexaToDec(hexaInput);
-        /*
-    Pass the string to the getHexaToDec function
-    and it returns the decimal form in the variable decOutput.
-         */
-        System.out.println("Number in Decimal: " + decOutput);
-        scan.close();
+        return val;
     }
 }

src/test/java/com/thealgorithms/conversions/HexaDecimalToDecimalTest.java

@@ -1,14 +1,37 @@
 package com.thealgorithms.conversions;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
-import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 public class HexaDecimalToDecimalTest {
 
-    @Test
-    public void testhexaDecimalToDecimal() {
-        assertEquals(161, HexaDecimalToDecimal.getHexaToDec("A1"));
-        assertEquals(428, HexaDecimalToDecimal.getHexaToDec("1ac"));
+    @ParameterizedTest
+    @CsvSource({
+        "A1, 161", // Simple case with two characters
+        "1AC, 428", // Mixed-case input
+        "0, 0", // Single zero
+        "F, 15", // Single digit
+        "10, 16", // Power of 16
+        "FFFF, 65535", // Max 4-character hex
+        "7FFFFFFF, 2147483647" // Max positive int value
+    })
+    public void
+    testValidHexaToDecimal(String hexInput, int expectedDecimal) {
+        assertEquals(expectedDecimal, HexaDecimalToDecimal.getHexaToDec(hexInput));
+    }
+
+    @ParameterizedTest
+    @CsvSource({
+        "G", // Invalid character
+        "1Z", // Mixed invalid input
+        "123G", // Valid prefix with invalid character
+        "#$%" // Non-hexadecimal symbols
+    })
+    public void
+    testInvalidHexaToDecimal(String invalidHex) {
+        assertThrows(IllegalArgumentException.class, () -> HexaDecimalToDecimal.getHexaToDec(invalidHex));
     }
 }