"cannot find symbol - class Scanner" error

This is my Code

public class Workshop3
{
    public static void main (String [] args)
    {
        System.out.println ("please enter radius of circle");
        double radius;
        Scanner keyboard = new Scanner (System.in);
        keyboard.nextDouble (radius);
    }
}

The error I recieve is

cannot find symbol - class scanner

on the line

Scanner keyboard = new Scanner (System.in);