如何强制用户为int输入固定数量的数字？

Question

I want to force the user to input a number with 5 lengths long ( no more and no less) and store them inside an int variable, this includes a leading 0.

For example, the program should allow the user to input:

12345
04123
00012

But it should not work for:

123456
4123
001

I tried...

  if(int x < 99999){
  //continue with code
  }

This would work only if the user input more than 5 lengths but it doesn't resolve the issue of the user putting in an int length less than 5

Answer 1

I think you should take input in string not in int then if the validation goes right you can parse it into integer like the following:

import java.util.Scanner;

public class main {

public static void main(String[] args) {
    /// take input
    String userInput = "";
    Scanner sc = new Scanner(System.in);
    userInput = sc.nextLine();
    int input ;
    // validation test
    if(userInput.length() == 5) {
        input = Integer.parseInt(userInput);
    }else {
        // you can display an error message to user telling him that he should enter 5 numbers!
    }
}

}

but you have to know that after parsing it into int if there's a leading zeros it could be gone.

Answer 2

Something crazy simple like this. Lacks edge case handling (Read: Negative values)

boolean matchesLength(int n, int lengthLim){
    char[] charArr = (n + "").toCharArray();
    return (charArr.length == lengthLim);
}

Answer 3

Two functions. First, to verify the input:

static boolean is_valid_number(String x) {
    // returns true if the input is valid; false otherwise
    if(x.length != 5 || Integer.valueOf(x) > 99999) {  
        // Check that both:
        //    - input is exactly 5 characters long
        //    - input, when converted to an integer, is less than 99999
        // if either of these are not true, return false
        return false;
    }
    // otherwise, return true
    return true;
}

and second, to get user input:

static int get_user_input() {
    // Create a scanner to read user input from the console
    Scanner scanner = new Scanner(System.in);
    String num = "";
    do {
        // Continuously ask the user to input a number
        System.out.println("Input a number:");
        num = scanner.next();
        // and continue doing so as long as the number they give isn't valid
    } while (!is_valid_number(num));
    return Integer.valueOf(num);

You might also have to do some error-handling, in case the given input isn't an integer at all. You could implement is_valid_number() concisely like this:

static boolean is_valid_number(String x) {
    try {
        return (x.length == 5 && Integer.valueOf(x) <= 99999);
    } catch (NumberFormatException e) {
        // Integer.valueOf(x) throws this when the string can't be converted to an integer
        return false;
    }
}