如何确保输入不是 null 并且只输入了一个字符？

Question

public static boolean correctchar(char b) {
    Scanner scan = new Scanner(System.in);
    b = scan.next().charAt(0);
    
    if (Character.toString(b).matches("^[a-zA-Z]") ) {
        System.out.println("True");
        return true;
    } else {
        System.out.println("False");
        return false;
    }
}

I have this method that checks whether the input is a letter in the alphabet or not, but I want to make sure that the input from the user is not null and that the user only enters one letter. For example " A " or " a " is a correct char, the problem is if I enter " Abcdef " then it is still true as the first letter is still a valid char. I want to make it so that the user can only enter one char, I think I've done that by using the scanner and charAt(0) but is there a more efficient way to do it, and I'm also not sure how to make it so that the input isn't null.

Answer 1

I've revised your code to do what you wanted:

public static boolean correctchar(char b) {
    Scanner scan = new Scanner(System.in);
    String input = scan.next();
    
    // This checks if the input is null, is empty (i.e., "") or is bigger than one character
    // If any of these conditions are fulfilled, then we return false.
    if (input == null || input.length() != 1) {
        return false;
    }

    b = input.charAt(0);
    
    if (Character.toString(b).matches("[a-zA-Z]") ) {
        System.out.println("True");
        return true;
    } else {
        System.out.println("False");
        return false;
    }
}

EDIT

Without scanner (see comments):

public static boolean correctchar(char b, String input) {        
    // This checks if the input is null, is empty (i.e., "") or is bigger than one character
    // If any of these conditions are fulfilled, then we return false.
    if (input == null || input.length() != 1) {
        return false;
    }

    b = input.charAt(0);
    
    if (Character.toString(b).matches("[a-zA-Z]") ) {
        System.out.println("True");
        return true;
    } else {
        System.out.println("False");
        return false;
    }
}

Answer 2

public static boolean correctChar() {
     try (Scanner scan = new Scanner(System.in)) {
         String input = null;
         do {
             input = scan.next();
             if (input != null && input.length() == 1) {
                 boolean isCorrect = input.matches("[a-zA-Z]"); 
                 System.out.println(isCorrect ? "True" : "False");
                 return isCorrect;
             } else {
                 System.out.println("Insert only one character");
             }
         } while (true);
     }
 }

Answer 3

I made couple of changes:

1. If invalid input ask user to enter again.

2. Make sure to close the scanner scan.close()

    Scanner scan = new Scanner(System.in); System.out.println("Please enter only one character: "); String input = scan.next(); while (null == input || input.isEmpty() || input.length() > 1) { System.out.println("Invaid Input, Please enter only one character: "); input = scan.next(); } scan.close(); if (Character.toString(input.charAt(0)).matches("^[a-zA-Z]")) { System.out.println("True"); return true; } else { System.out.println("False"); return false; }

   }