```/***************************************************

  Name: Gorm Osborg
  Date: 17/2/2017
  Homework #7

  Program name:        HexConversion
  Program description: Accepts hexadecimal numbers as input and converts each number to binary
                       and to the decimal equivalent.
                       Valid input examples: F00D, 000a, 1010, FFFF, Goodbye, GOODBYE
                       Enter GOODBYE (case insensitive) to exit the program.

****************************************************/

package hw8;

import java.util.Scanner;

public class HexConversion {

  public static void main(String[] args) {

    // Maximum length of input string
    final byte INPUT_LENGTH = 4;

    String userInput = "";                   // Initialize to null string
    Scanner input = new Scanner(System.in);

    // Process the inputs until GOODBYE is entered
    do {
      // Input a 4 digit hex number
      System.out.print("\nEnter a hexadecimal string, or enter GOODBYE to quit:  ");
      userInput = input.next().toUpperCase();

      // Process the input
      switch (userInput) {

        case "GOODBYE": break;

        default:        if (isValidHex(userInput, INPUT_LENGTH)) {
                          // The input is a valid hexadecimal string

                          // Convert the hexadecimal string to binary and print the binary number as a string
                          String binVal = hex2Bin(userInput, INPUT_LENGTH);

                          // Convert the hexadecimal string to decimal and print the decimal number
                          long decVal = hex2Dec(userInput, INPUT_LENGTH);
                          System.out.printf("      0x%s = %s in binary = %d in decimal (unsigned).\n", userInput, binVal, decVal);
                        }

                        else {
                          // String is either the wrong length or is not a valid hexadecimal string
                          System.out.printf("      The string %s is not a valid input.\n", userInput);
                        }
                        break;
        }
    } while (!userInput.equals("GOODBYE"));

    // Exit the program
    System.out.println("\nGoodbye!");
    input.close();
  }

  // Method to validate the input
  public static boolean isValidHex(String userIn, byte inputLen) {
    boolean isValid = false;

    // If length of the input string is equal to inputLen, continue with validation,
    // otherwise return false
    if (userIn.length() == inputLen) {

      // The length is correct, now check that the characters are legal hexadecimal digits
      for (int i = 0; i < inputLen; i++) {
        char thisChar = userIn.charAt(i);

        // Is the character a decimal digit (0..9)? If so, advance to the next character
        if (Character.isDigit(thisChar)) {
          isValid = true;
        }

        else {
          // Character is not a decimal digit (0..9), is it a valid hexadecimal digit (A..F)?
          if ("ABCDEFabcdef".indexOf(thisChar) != -1) {	// Can just make the string "0123456789ABCDEFabcdef" and simplify most of this method
        	  isValid = true;
          }
          else;
          	break;
        }
        break;
      }
    }

    // Return true if the string is a valid hexadecimal string, false otherwise
    if (isValid == true) {
    	return true;
    }
    else {
    	return false;
    }
  }


  // Method to convert the hex number to a binary string
  public static String hex2Bin(String hexString, byte inputLen) {
    String binString = "";     // Initialize binString to null string

    // Convert each hexadecimal digit to its binary equivalent
    for (int i = 0; i < inputLen; i++) {
      char thisChar = hexString.charAt(i);
      if (thisChar.isDigit %2 == 0) {
    	  thisChar = 1;
      }
      else;


      // Convert hexString to a binary string, e.g. F00D = 1111000000001101
      TBD
    }
  }

  // Method to convert the hex number to decimal number
  public static long hex2Dec(String hexString, byte inputLen) {

    // Convert hexadecimal string to decimal, e.g. F00D = 61453 in unsigned decimal
    TBD
  }
}
```