See, String is coool

import java.util.Scanner;

public class Main {

    public static void main(String[] args)
    {
        Scanner keyboard = new Scanner(System.in);
        // set up Scanner for keyboard input


        // Read in phone number
        System.out.println("Enter a 10-digit phone number without spaces");
        String fullPhone = keyboard.next();

        // Find area code part
        String areaCode = fullPhone.substring(0,3);

        // Find last 4 digits
        String lastFour = fullPhone.substring(6);
        //fullPhone = 10000 - exchange;

        // Find exchange part
        String exchange = fullPhone.substring(3,6);


        // Print original, plus new number all formatted
        System.out.println("Original number entered: " + fullPhone);
        System.out.println("Phone number formatted: ("+ areaCode + ")" + exchange + "-" + lastFour);
    }

}