Enhance docs, remove main. add more tests in `HexaDecimal… (#5923)

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();
-    }
 }