Conversion from binary to octal in java

Java program for Conversion from binary to octal. Here mentioned other language solution.

/*
  Java program for
  Convert Binary number into Octal number
*/
public class Convert
{
    // Convert decimal number into octal
    public int octal(int number)
    {
        int result = 0, multiplier = 1;
        int remainder = 0;
        while (number != 0)
        {
            remainder = number % 8;
            result = (remainder * multiplier) + result;
            multiplier *= 10;
            number = number / 8;
        }
        return result;
    }
    public void binaryToOctal(String num)
    {
        if (num.length() == 0)
        {
            // When empty binary number
            return;
        }
        // Some useful variable
        boolean flag = false;
        int decimalNo = 0, counter = 0;
        int index = num.length() - 1;
        // We Assume that given binary number is valid
        // Here - indicates negative binary number
        // First convert binary to decimal
        // Example = 10111 => 23
        while (index >= 0)
        {
            if (num.charAt(index) == '1')
            {
                decimalNo += (1 << counter);
            }
            else if (num.charAt(index) != '0')
            {
                if (index == 0 && num.charAt(index) == '-')
                {
                    // When get negative number
                    flag = true;
                }
                else
                {
                    // Not a valid binary number
                    return;
                }
            }
            counter += 1;
            index -= 1;
        }
        // When given number is 
        int output = octal(decimalNo);
        if (flag == true)
        {
            output = -output;
        }
        // Display given number
        System.out.print("Binary : " + num);
        // Display result
        System.out.println(" Octal : " + output);
    }
    public static void main(String[] args)
    {
        Convert task = new Convert();
        // Test Case
        task.binaryToOctal("1111");
        task.binaryToOctal("10111");
        task.binaryToOctal("101011");
        task.binaryToOctal("11011");
        task.binaryToOctal("-1000110");
    }
}

Output

Binary : 1111 Octal : 17
Binary : 10111 Octal : 27
Binary : 101011 Octal : 53
Binary : 11011 Octal : 33
Binary : -1000110 Octal : -106