Luhn check for Credit Card Validation code

My code doesn't work, and I'm looking to validate credit card numbers using the Luhn check. I'm stuck and unsure what else it is that i need to do.

package validatecc1;
import java.util.*; // Imports objects such as Scanner.

public class ValidateCC1 {

    public static final int INVALID = -1;
    public static final int VISA = 0;
    public static final int MASTERCARD = -1;
    public static String [] cardNames = 
{
    "Visa Card",
    "Mastercard Card",
};
    
  // Method to test that digits are only entered.
    
      private static String getDigitsOnly (String s) 
      {
        StringBuffer digitsOnly = new StringBuffer ();
        char c;
        for (int i = 0; i < s.length (); i++) 
        {
            c = s.charAt (i);
            if (Character.isDigit (c)) 
            {
                digitsOnly.append (c);
            }
        }
        return digitsOnly.toString ();
      }
      
    // Method to run the Luhn check.
      
public static boolean isValid(String cardNumber)
{
 String digitsOnly = getDigitsOnly(cardNumber);
        int sum = 0;
        int digit = 0;
        int addend = 0;
        boolean timesTwo = false;
        for (int i = digitsOnly.length () - 1; i >= 0; i--) {
            digit = Integer.parseInt (digitsOnly.substring (i, i + 1));
            if (timesTwo) {
                addend = digit * 2;
                if (addend > 9) {
                    addend -= 9;
                }
            } else {
                addend = digit;
            }
            sum += addend;
            timesTwo = !timesTwo;
        }
        int modulus = sum % 10;
        return modulus == 0;
}

  // Method to find the Sum of Even places in reverse.

public static int sumOfDoubleEvenPlace(String cardNumber)
{
  int result = 0;
  long start = 0;
  String digits = Long.toString(number);
  if ((digits.length() % 2) == 0) 
  {
   start = digits.length() - 1; 
  }
    else 
  {
    start = digits.length() - 2; 
  }
      while (start != 0) 
      {
        result += (int) ((((start % 10) * 2) % 10) + (((start % 10) * 2) / 2));
        start = start / 100;
}
        return result;  
}

  // Method to 

public static int getDigit(int number)
{
    return number % 10 + (number / 10);
}
public static int sumOfOddPlace(String cardNumber)
{
  int result = 0;
  while (number != 0) 
  {
    result += (int) (number % 10);
    number = number / 100;
   }
    return result;   
}
public static boolean prefixMatched(long number, int d)
{
    return getPrefix(number, getSize(d)) == d;
}
public static int getSize(long d)
{
     int numberOfDigits = 0;
     String sizeString = Long.toString(d);
     numberOfDigits = sizeString.length();
     return numberOfDigits;
}
public static long getPrefix(long number, int k)

{
 String size = Long.toString(number);
   if (size.length() <= k)
    {
     return number; 
    }
      else
{    
    return Long.parseLong(size.substring(0, k));
}
}

    public static void main(String[] args) {
     Scanner input = new Scanner(System.in);
     System.out.print("Enter a credit card number as a long integer: ");
     long number = input.nextLong();
     if (isValid(number)) 
   {
     System.out.println(number + " is valid");
   }
     else 
   {
     System.out.println(number + " is invalid");
   }
   }
}