public String intToComplement(int num) {
    StringBuilder sb = new StringBuilder();
    if (num >= 0) {
        for (int i = 0; i < 8; i++) {
            int temp = num % 16;
            if (temp >= 10) {
                sb.append((char) (temp - 10 + 'a'));
            }
            else {
                sb.append(temp);
            }
            num /= 16;
        }
    }
    else {
        int[] a = new int[32];
        a[0] = 1;
        num = -num;
        //求出原码
        for (int i = 0; i < 31; i++) {
            int temp = num % 2;
            a[31 - i] = temp;
            num /= 2;
        }
        //求反码
        for (int i = 0; i < 31; i++) {
            if (a[31 - i] == 1) {
                a[31 - i] = 0;
            }
            else {
                a[31 - i] = 1;
            }
        }
        //求补码
        int carry = 1;
        for (int i = 0; i < 31; i++) {
            int x = (carry + a[31 - i]) % 2;
            carry = (carry + a[31 - i]) / 2;
            a[31 - i] = x;
        }
        for (int i = 0; i < 8; i++) {
            int y = 0;
            for (int j = 0; j < 4; j++) {
                y += a[31 - 4 * i - j] * (int) Math.pow(2, j);
            }
            if (y >= 10) {
                sb.append((char)(y - 10 + 'a'));
            }
            else {
                sb.append(y);
            }
        }
    }
    sb.append("x0");
    sb.reverse();
    return sb.toString();
}

附带一个二进制、十进制、十六进制的表格