Why must the part throws IOException be used?

A good answer might be:

This informs the compiler to prepared for an input operation that might fail.

Say that you want to write a program that calculates the total bill for a restaurant meal. The input to the program will be the basic cost of the meal. To this is added 6% sales tax and a 20% tip on the before-tax cost. Arithmetic will be done with type double. Here is the program, with some parts left out:

import java.io.*;
class RestaurantBill
{
  public static void main (String[] args) 
      ___________________________

  {
    String charData;
    double basicCost;

    __________________________________

    __________________________________

    __________________________________

    __________________________________

    System.out.println("basic cost: " + 
        basicCost + " total cost: " 
        + ________________________ );
  }
}

These are the missing parts, but not in order:

(basicCost + basicCost*0.06 + basicCost*0.20)

System.out.println("Enter the basic cost:");

basicCost  = Double.parseDouble( charData  ) ;

BufferedReader stdin 
    = new BufferedReader( 
      new InputStreamReader( System.in ) );

charData = stdin.readLine();

throws IOException

QUESTION 6:

Fill in the missing parts.