How can I modify my credit card validator program to check only for valid numbers in my last function that check the card type?

Question

public class prog {

// Global variables to be use in the whole program
public static int numOfInvalids = 0 , visaNum=0 ; 



public static void main(String[] args) 
{
    
    Scanner userInput = new Scanner(System.in) ;
    

    System.out.println("How many credit card you want to check?");
    int numOfCredit = userInput.nextInt() ;
    
    int creditnumbers[] = new int[numOfCredit] ;
    
    for (int i = 0 ; i<numOfCredit ; i++)
    {
        System.out.println("Enter credit card number "+(i+1)+": ") ;
        String creditNumber = userInput.next() ;
        
        // checks if number is a credit number or not
        
        if (creditNumber.length()<16 || creditNumber.length()>19)            
        {
            System.out.println("Not a credit card number!") ;
            prog.numOfInvalids ++ ;  // count number of invalid card numbers
            
            System.out.println();
            continue ;
        }
        
        checkValid(creditNumber);
        checkType(creditNumber) ;
        // calls function 

    }
    System.out.println("Number of invalid: "+prog.numOfInvalids) ;
    System.out.println("Number of visa credit card: "+prog.visaNum) ;
    
    
}



private static boolean checkValid(String cardNumber)        // function to check if card number is valid or not 
{
    int cardlength = cardNumber.length();
    int evenSum = 0, oddSum = 0, sum;
    
    
    for (int i = cardlength - 1; i >= 0; i--)  //looping from the right 
    {
        int digit = Character.getNumericValue(cardNumber.charAt(i));  // Character.getNumericValue = convert char to integer
        
        if (i % 2 == 0)  // taking every 2nd digits from the right e.g 2nd..4th..6th..8th etc.
        {
            int multiplyByTwo = digit * 2;     // multiply each 2nd digits by 2
            
            if (multiplyByTwo > 9)  // Add two digits to handle cases that make two digits after doubling  e.g (6 * 2 = 12) = 1 + 2
            {
                
                String doubleDigits = String.valueOf(multiplyByTwo);  // convert double digit to String for use of charAt function
                multiplyByTwo = Character.getNumericValue(doubleDigits.charAt(0)) + Character.getNumericValue(doubleDigits.charAt(1));   //convert each character to integer and add them together
            }
            
            evenSum += multiplyByTwo;  // add product digits together
            
            } 
        
        else 
        {
            oddSum += digit;      // Add sum of digits that weren’t multiplied by 2
        }
    }
    
    sum = evenSum + oddSum;   //Adding both sums together
    
    if (sum % 10 == 0)       //if sum is multiple of 10 , it is valid 

    {
        
        System.out.println("valid card");
        System.out.println();
        return true;    
       
    } 
    
    else
    {
        System.out.println("invalid card") ; 
        prog.numOfInvalids ++ ; // count the number of invalid card numbers
        System.out.println();
        return false;
        
    }
}

private static boolean checkType(String cardNumber) // function to check card type 
{
    


    if ( (cardNumber.charAt(0)=='4') && cardNumber.length()==13 || cardNumber.length()==16 )  //checking first character and length of card number
            {
                System.out.println("VISA");
                visaNum ++ ; // counting number of visa cards 
                return true;    
            } 
            
            else
            {
           
                return false;
            }

} }

I want to check the card type for valid cards only from the first function "check Valid" into my last function "Check Type "

Is there any way I can save the valid numbers from the first function and use them in the "Check Type " function?

I want to check the card type for valid cards only from the first function "check Valid" into my last function "Check Type "

Is there any way I can save the valid numbers from the first function and use them in the "Check Type " function?

Answer 1

Assumptions based on the question

1. Need to infer card type only for valid cards
2. Function return type can not be changed

Possible approach

1. Add a if check on the response of checkValid and if its true, then call checkType

   if (checkValid(creditNumber) {
        checkType(creditNumber);
   }

Suggestion

1. Irrespective of this solution, please improve the formatting while coding and asking questions as it will help others to better understand the problem.