Conversion from binary to octal in scala

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

import scala.collection.mutable._;
//  Scala program for
//  Convert Binary number into Octal number
class Convert ()
{
    // Convert decimal number into octal
    def octal(num : Int) : Int=
    {
        var result = 0
        var multiplier = 1
        var remainder = 0
      	var number = num
        while (number != 0)
        {
            remainder = number % 8
            result = (remainder * multiplier) + result
            multiplier *= 10
            number = number / 8
        }
        return result
    }
    def binaryToOctal(num : String) : Unit=
    {
        if (num.length() == 0)
        {
            // When empty binary number
            return
        }
        // Some useful variable
        var flag = false
        var decimalNo = 0
        var counter = 0
        var 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 
        var output = octal(decimalNo)
        if (flag == true)
        {
            output = -output
        }
        // Display given number
        print("Binary : " + num)
        // Display result
        println(" Octal : " + output)
    }
}

object Main 
{
	def main(args : Array[String]) : Unit=
    {
        var 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