Thread: anyone can tell me how this code works?

public void addBinary(String a, String b) throws IOException{

StringBuilder sb = new StringBuilder();
int i=a.length()-1,j= b.length()-1, carry = 0;

while (i>=0 || j>=0){
int suma = carry;
if(j >=0) {
suma = suma + b.charAt(j--) - '0';
}
if(i >=0) {
suma = suma + a.charAt(i--) - '0';
}
sb.append(suma % 2);
carry= suma / 2;
}
if (carry!= 0){
sb.append(aca);
}
}

this code works perfectly. but i can understand how this logic works ( suma = suma + b.charAt(j--) - '0'; )
can someone can explain me ? thank you folks
sorry for my english

thanks. this code work fine, aca is carry, i forget to change it
thank you for the answer