Checksum for sending data

import java.util.Scanner;
class BinaryAdd{
    static String complement(String str){
        String s1 = "";
        for(int i = 0; i < 8; i++)
            if(str.charAt(i) == '1') s1 += "0";
            else s1 += "1";
        return s1;
    }
    static String appendZeros(String str){
        for(int i = 8 - str.length(); i != 0; i--) str = "0" + str;
        return str;
    }
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter an 16-bit binary number: ");
        String data = sc.next();
        int n1 = Integer.parseInt(data.substring(0,8), 2);
        int n2 = Integer.parseInt(data.substring(8), 2);
        String sum = appendZeros(Integer.toBinaryString(n1 + n2));
        String toSend = data + complement(sum);
        System.out.println(toSend +" is being sent(data+checksum).");
        n1 = Integer.parseInt(toSend.substring(0,8), 2);
        n2 = Integer.parseInt(toSend.substring(8, 16), 2);
        int n3 = Integer.parseInt(toSend.substring(16), 2);
        sum = appendZeros(Integer.toBinaryString(n1 + n2 + n3));
        if(complement(sum).equals("00000000")) System.out.println("Success.");
        else System.out.println("Failed.");
    }
}