BonusQuestion4

import java.util.Scanner;

public class BonusQuestion4 {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);

        System.out.print("Enter a four-digit binary string: ");
        String binaryString = input.nextLine();

        if (binaryString.length() != 4) {
            System.out.println("Incorrect input size");
            System.exit(1);
        }

        int decimal = 0;
        for (int i = 0; i < 4; i++) {
            if (binaryString.charAt(i) != '0' && binaryString.charAt(i) != '1') {
                System.out.println("Input must be only be 0s and 1s");
                System.exit(1);
            } else {
                int exp = binaryString.length() - (i + 1); // Ex: 4 - (0 + 1) = 3
                decimal += (binaryString.charAt(i) - '0') * Math.pow(2, exp);
            }
        }

        System.out.println("The decimal number for " + binaryString + " is " + decimal);
    }
}