BooleanComplementer.java

import java.util.Scanner;

public class BooleanComplementer {

    /*
       Takes a string as in input and returns a string with a boolean complement
       of each digit except the last one. Example: 11101 returns 00011
    */
    public static String getComplement (String inp) {
        String ans = "";
        // loop through each character of inp
        for (int i=0; i<inp.length(); i++) {
            if (!(inp.substring(i, i+1).equals("1") || inp.substring(i, i+1).equals("0"))) {
                System.out.println ("Not a binary string!");
                ans = "-1";
                break;
            }
            if (i<inp.length()-1) {
                // when input character is 1, add 0 and vice versa
                ans += (inp.substring(i, i+1).equals("1")? "0": "1");
            } else {
                // last digit remains unchanged
                ans += inp.substring(i, i+1);
            }
        }
        return ans;
    }

    public static void main (String[] args) {
        Scanner in = new Scanner (System.in);
        String input, answer;
        System.out.print ("Enter boolean string: ");
        input = in.nextLine();
        answer = getComplement (input);
        if (!answer.equals ("-1")) {
            System.out.println ("Answer: " + answer);
        }
    }
}