Java converting negative binary back to integer

Question

I'm trying to convert an base 10 number to a base 2 and back to base 10. It works only for positive argument_decimal

argument_binary = Integer.toBinaryString(argument_decimal);
back_converted_argument_decimal = Integer.valueOf(argument_binary, 2);

For argument_decimal beeing negative, I get "java.lang.NumberFormatException: For input string: "11111111111111111111111111111111""

EDIT: here is what I do:

latitude_binary = Integer.toBinaryString((int)(latitude_decimal * 1000000));   
back_converted_latitude_decimal =  Long.parseLong(latitude_binary, 2) / 1000000.0;

which gives me bad results like -1.1 being forth and back converted to 4293.867296

Answer 1

Try to go via a long:

String binary = Integer.toBinaryString(-1);
long l = Long.parseLong(binary, 2);
int i = (int) l;

Tested, and working.

Why this works is because -1 is represented as a sequence of 32 bits 1 in system memory. When using the toBinaryString method, it creates a string using that exact representation. But, 32 bits of one is in fact equal to 2^32 - 1. That is too large for an int (4 bytes), because an int goes from [-2^31, 2^31-1]. This is because the most left bit is representing the sign. So to fix that overflow, first interpret that sequence of 1 and 0 characters as a Long. A long will do because the maximum value for a long is 2^63-1. Then convert the long to an int. This is done by simply taking the lower 32 bits.

---

The bug in your code is that you didn't cast the Long.parseLong to an int. So this should work:

lat_bin = Integer.toBinaryString((int)(lat_dec * 1000000));   
lat_dec_conv =  ((int) Long.parseLong(lat_bin, 2)) / 1000000.0;

Answer 2

 public static void convertStringToDecimal(String binary) {
    int decimal = 0;

    int power = 0;

    if (binary.charAt(0) == '1' && binary.length() == 32) {

        StringBuilder builder = new StringBuilder();

        for (int i = 0; i < binary.length(); i++) {

            builder.append((binary.charAt(i) == '1' ? '0' : '1'));

        }

        while (binary.length() > 0) {

            int temp = Integer
                    .parseInt(builder.charAt((binary.length()) - 1)+"");
            decimal += temp * Math.pow(2, power++);
            binary = binary.substring(0, binary.length() - 1);

        }

        System.out.println((decimal + 1) * (-1));

    } else {

        while (binary.length() > 0) {
            int temp = Integer
                    .parseInt(binary.charAt((binary.length()) - 1) + "");
            decimal += temp * Math.pow(2, power++);
            binary = binary.substring(0, binary.length() - 1);
        }

        System.out.println(decimal);

    }
}