Luhn — SteemCN

class LuhnValidator {
boolean isValid(String candidate) {
    int totalsum = 0;
    candidate = candidate.replace(" ", "");
    if(candidate.length() <=1) return false;
    for(int n = candidate.length()-1; n >= 0; n--){
        if(!Character.isDigit(candidate.charAt(n))) return false;
        if(candidate.length() % 2==0){
            if(n % 2 ==0){
                int newCandidate = Character.getNumericValue(candidate.charAt(n));
                newCandidate *=2;
                if(newCandidate > 9) newCandidate -=9;
                totalsum += newCandidate; 
            }
           else{
                int newCandidateEven = Character.getNumericValue(candidate.charAt(n));
                totalsum += newCandidateEven;
            } 
        }
        else{
                if(n % 2 !=0){
                    int newCandidate = Character.getNumericValue(candidate.charAt(n));
                    newCandidate *=2;
                    if(newCandidate > 9) newCandidate -=9;
                    totalsum += newCandidate; 
            }
               else{
                    int newCandidateEven = Character.getNumericValue(candidate.charAt(n));
                    totalsum += newCandidateEven;
            }   
        }
    }  
    return totalsum % 10==0;
}
}