static void luhn(){
    System.out.print("Enter number to validate:\n");
    String pnr = input.nextLine();
    int length = pnr.length();
    int sum = 0;
    for (int i = 1, pos = length - 1; i < 10; i++, pos--){
        char tmp = pnr.charAt(pos);
        int num = tmp - 0
        int product;
        if (i % 2 != 0){
            product = num * 1;
        }
        else{
            product = num * 2;
        }
        if (product > 9)
            product -= 9;
        sum+= product;              
        boolean valid = (sum % 10 == 0);
        if (valid){
            System.out.print("Valid!\r");
        }
        else{
            System.out.print("Invalid!");
        }
    }
}

我看到的第一件事是您拥有：

int num = tmp - 0

您应该改为：

int num = tmp - '0';

其次，您应该在循环 外 验证和for，因为您只关心处理完所有数字后的和。

第三，您从数字的末尾开始，并且不包括字符串的第一个数字。为什么不同时使用i这两个任务？

结果（工作）方法：

static void luhn(){
  System.out.print("Enter number to validate:\n");
  String pnr = input.nextLine();
  // this only works if you are certain all input will be at least 10 characters
  int extraChars = pnr.length() - 10;
  if (extraChars < 0) {
    throw new IllegalArgumentException("Number length must be at least 10 characters!");
  }
  pnr = pnr.substring(extraChars, 10 + extraChars);
  int sum = 0;
  // #3: removed pos
  for (int i = 0; i < pnr.length(); i++){
    char tmp = pnr.charAt(i);
    // #1: fixed the '0' problem
    int num = tmp - '0';
    int product;
    if (i % 2 != 0){
      product = num * 1;
    }
    else{
      product = num * 2;
    }
    if (product > 9)
      product -= 9;
    sum+= product;              
  }
  // #2: moved check outside for loop
  boolean valid = (sum % 10 == 0);
  if (valid){
    System.out.print("Valid!\r");
  }
  else{
    System.out.print("Invalid!");
  }
}

---

从风格上讲 ，如果代替方法签名，则此方法会更有用

static void luhn() {

相反，它具有方法签名

static boolean luhn(String input) {

这很容易让您的代码String从任何来源（文件，硬编码等）获取，并对结果做任何事情（像打印消息一样打印消息，或执行其他操作）。很明显，你会移动System.out.print，input.nextLine()以及if(valid)这种方法的代码之外的位。

完整的重构程序：

import java.util.Scanner;

public class Luhn {
  private static Scanner input;

  public static void main(String... args) {
    input = new Scanner(System.in);
    System.out.print("Enter number to validate:\n");
    String pnr = input.nextLine();
    boolean result = luhn(pnr);
    printMessage(result);
    input.close();
  }

  static boolean luhn(String pnr){
    // this only works if you are certain all input will be at least 10 characters
    int extraChars = pnr.length() - 10;
    if (extraChars < 0) {
      throw new IllegalArgumentException("Number length must be at least 10 characters!");
    }
    pnr = pnr.substring(extraChars, 10 + extraChars);
    int sum = 0;
    for (int i = 0; i < pnr.length(); i++){
      char tmp = pnr.charAt(i);
      int num = tmp - '0';
      int product;
      if (i % 2 != 0){
        product = num * 1;
      }
      else{
        product = num * 2;
      }
      if (product > 9)
        product -= 9;
      sum+= product;              
    }
    return (sum % 10 == 0);
  }

  private static void printMessage(boolean valid) {
    if (valid){
      System.out.print("Valid!\r");
    }
    else{
      System.out.print("Invalid!");
    }
  }
}