Converting 32 bit hexadecimals to decimal in java

for an assignment I have to write a program that will take in an 8 character string(hexadecimal) and then convert it to base 10. I am not allowed to use any outside classes to do this. I'm pretty sure I have it working properly... for positive numbers only. My problem is how to show negative numbers. an example is that FFFFFFFA should print out as -6 This is my code so far

package hexconverter;

import java.util.*;

/**
 *
 * @author Steven
 */
public class Main {

    Scanner scanner = new Scanner(System.in);

    public void doWork() {



        System.err.println("Please enter the internal representation: ");
        String hex;
        hex = scanner.next();
        hex = hex.toUpperCase();

        long count = 1;
        long ans = 0;

        for (int i = 7; i >= 0; i--) {
            Character c = hex.charAt(i);

            if (c != '1' && c != '2' && c != '3' && c != '4' && c != '5' && c != '6' && c != '7' && c != '8' && c != '9') {
                int num = fixLetters(c);
                ans = ans + (num * count);
                count = count * 16;
            } else {

                String s = c.toString(c);
                long num = Integer.parseInt(s);
                ans = ans + (num * count);
                count = count * 16;
            }
        }

       if (ans > 2147483647) {
            System.out.println("is negative");


       } else {
            System.out.println(ans);
       }
    }

    public int fixLetters(Character c) {
        if (c.equals('A')) {
            return 10;
        } else if (c.equals('B')) {
            return 11;
        } else if (c.equals('C')) {
            return 12;
        } else if (c.equals('D')) {
            return 13;
        } else if (c.equals('E')) {
            return 14;
        } else if (c.equals('F')) {
            return 15;
        } else {
            return 0;
        }

    }

    public static void main(String[] args) {
        // TODO code application logic here
        Main a = new Main();
        a.doWork();
    }
}

I think my test for a negative integer is correct... since that is the highest value 32 bits can hold, anything over that will be an overflow so that means it should be negative. 开发者_开发技巧From here I have no idea how to go about this. Any pointers or tips would be greatly appreciated. If there is no way to do it mathematically I feel like I am going to have to convert the hex into binary and then perform twos complements on it, but again I dont know where to start.

Thanks in advance

If the number is negative (> 2147483647) in your code, just subtract 2^32 (4294967296) from it. Then print it out.

if (ans > 2147483647) {
        System.out.println(ans - 4294967296L);
   } else {
        System.out.println(ans);
   }

In 32-bit 2's complement binary representation, the value of a negative is exactly 2 ^ 32 less than the value of the same bit pattern in unsigned representation. You have already identified that the number might be negative; all that's left to do is subtract 2 ^ 32.

Of course, 2 ^ 32 (4294967296 in decimal, or 0x100000000 in hex) is a value that can't be represented by Java's "int" type, so you'll need to use a "long":

if (ans > 2147483647) {
    // System.out.println("is negative");
    ans = ans - 0x100000000L;
    System.out.println(ans);
} else {