code: public static int assertIsPositive(int num, String fieldName) {
          if (num <= 0) {
              throw new IllegalArgumentException(String.format("%s must be positive", fieldName));
          }
          return num;
      }
docs: Asserts that the given number is positive (non-negative and non-zero).
  
  @param num       Number to validate
  @param fieldName Field name to display in exception message if not positive.
  @return Number if positive.