import java.util.*;				// for Scanner

public class Ch14_6				// this program converts a hexadecimal number into decimal throwing a NumberFormatException if the characters of the input are not legal for a hexadecimal number (0-9, A-F are legal)
{
	public static void main(String[] args)
	{
		Scanner s=new Scanner(System.in);
		String input="";					// variables used in try and finally are declared here
		boolean correct=true;				// assume user will enter a valid hexadecimal number
		int value=0;						// resulting decimal value
		try{
			System.out.println("Enter a hex value and I will convert it to decimal");
			input=s.next();					// user's hex value
			char temp;						// we can declare temp here because it is not used in the catch or finally clauses
			for(int i=0;i<input.length();i++)		// for each character in the user's input
			{
				temp=input.charAt(i);				// get that character and convert it from hex to decimal or throw an exception if not legal
				switch (temp){
					case '0': value=value*16;break;		// the conversion works like this:  for each digit in order from right to left, convert it to decimal and then
					case '1': value=value*16+1;break;	// for each successive digit, multiply the previous converted portion by 16
					case '2': value=value*16+2;break;
					case '3': value=value*16+3;break;	// example:  9A31:  9*16^3 + A(10)*16^2 + 3*16^1 + 1*16^0 (16^0 = 1) = 9*4096 + 10*256 + 3*16 + 1*1 = 36864 + 2560 + 48 + 1 = 39473
					case '4': value=value*16+4;break;	// we get this value as follows:  value = 0, temp = 9, value = 0 * 16 + 9 = 9
					case '5': value=value*16+5;break;	//								  value = 9, temp = A, value = 9 * 16 + 10 = 154
					case '6': value=value*16+6;break;	//								  value = 154, temp = 3, value = 154 * 16 + 3 = 2467
					case '7': value=value*16+7;break;	//								  value = 2467, temp = 1, value = 2467 * 16 + 1 = 39473
					case '8': value=value*16+8;break;
					case '9': value=value*16+9;break;
					case 'A': value=value*16+10;break;
					case 'B': value=value*16+11;break;
					case 'C': value=value*16+12;break;
					case 'D': value=value*16+13;break;
					case 'E': value=value*16+14;break;
					case 'F': value=value*16+15;break;	// if temp is not 0-9 or A-F, then illegal character, throw a NumberFormatException
					default: throw new NumberFormatException("In String " + input + ", the character " + temp + " is not a legal Hexadecimal digit");
				}
			}
		}
		catch(NumberFormatException e)
		{
			System.out.println(e);		// output the exception
			correct=false;				// and flip correct to false so that in finally, we output that value could not be computed
		}
		finally {				// output computed value or message that the input was illegal
			if(correct) System.out.println("The hexadecimal value " + input + " converts to the decimal value " + value);
			else System.out.println("Could not convert this illegal input into decimal");
		}
	}
}