在整数Java上，Character.IsDigit返回false而不是true

Question

Hi I am newer to programming so I need some help. I need to make a method where I check if a birthday is correct. For instance 960214 where 96 is year 02 is month and 14 is day, but the birthday is a String. So here is what I got:

private static boolean checkCorrect(String a) {

    int year = Integer.parseInt(a.substring(0,2));

    int month = Integer.parseInt(a.substring(2, 4));

    int day = Integer.parseInt(a.substring(4, 6));

    if (a.length() == 6 && Character.isDigit(year)) {
        return true;
    } else
        return false;
}

Now I stopped at Character.isDigit(year) because it returns false where it should return true. I printed year just to see what comes out and 96 comes out just like in the example on top. What am I doing wrong?

Answer 1

Character.isDigit(year) excepts a char not a number. Character.isDigit('5') this will return true.

Answer 2

import java.util.Scanner;

class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    System.out.print("Write your Birthday in the form yymmdd: ");
    String date = input.nextLine();
    if(checkDate(date))
      System.out.println("Your Birthday: " + date + " is valid :)");
    else
      System.out.println("Your Birthday: " + date + " is invalid :(");
  }

  public static Boolean checkDate(String date){
    if(date.length() != 6) {
      System.err.println("Please enter your Birthday in the form yymmdd... ");
      System.exit(1);
    }
    try {
      int year = Integer.parseInt(date.substring(0,2));
      int month = Integer.parseInt(date.substring(2, 4));
      int day = Integer.parseInt(date.substring(4, 6));
      if ((00<=year && year<=99) && (01<=month && month<=12) && (01<day && day<=31)) 
        return true;
    } catch (NumberFormatException e) {
        e.printStackTrace();
    }
    return false;
  }
}

Try it here!

Answer 3

Note:

If I understand correctly, you want to make sure that the inputted String is a number. The following code should if that is the issue.

Advice:

You should check if it is a number before parsing the Integer, because if you parse it immediately and it is invalid, it will cause an Exception.

Code:

public static void main(String args[]) {
    String input = "960214"; // Sets the value of the String
    String year = input.substring(0, 2); // Finds the "year" value in the
                                            // input
    Boolean isANumber = true; // The year is thought to be a number unless
                                // proven otherwise
    try {
        Integer.parseInt(year); // Tries to parse the year into an integer
    } catch (Exception ex) { // Catches an Exception if one occurs (which
                                // would mean that the year is not an
                                // integer
        isANumber = false; // Sets the value of "isANumber" to false
    }
    if (isANumber) { // If it is a number...
        System.out.println(year + " is a number!"); // Say it is a number
    } else {
        System.out.println(year + " is not a number!"); // Say it is not a
                                                        // number
    }
}

Output:

96 is a number!

Output (When the "input" is "xx0214"):

xx is not a number!