Stopping a repeating Println in a while loop, BlueJ

Question

Below is the code I have so far, it is a calculator for my computer science class. The problem I am having is that on the second run of the program the System.out.print("Would you like to perform a calculation? (y/n) "); runs twice instead of once. I have already turned in the project, but I would like to know why it does this and how, in my future programs, I can fix it. I'll post the rest of the code below.

I appreciate all of your help, I credit the "A" that I got on it to all of you. Thanks!

/**
* A calculator with multiple functions.
* @author () 
* @version (version 2.1)
*/
import java.io.*;
import java.util.Scanner;
public class Calc
{
  public static void main(String args[])
  {
    Scanner reader = new Scanner(System.in);
    String cont = "", funct = "";
    double fnum = 0, snum = 0, answer = 0;

    while(true)
    {
        System.out.print("Would you like to perform a calculation? (y/n) ");
        cont = reader.nextLine();
        if (cont.equalsIgnoreCase("y"))
        {
            System.out.println("What function would you like to do?");
            System.out.println("+?");
            System.out.println("-?");
            System.out.println("*?");
            System.out.println("/?");
            funct = reader.nextLine();
            if (funct.equals("+"))
            {
                System.out.println("Simple addition calculator");
                System.out.println("Enter first num: ");
                fnum = reader.nextDouble();
                System.out.println("Enter second num: ");
                snum = reader.nextDouble();
                answer = fnum + snum;
                System.out.println("Answer: " + answer);
                System.out.println(" ");
            }
            if (funct.equals("-"))
            {
                 System.out.println("Simple subtraction calculator");
                System.out.println("Enter first num: ");
                fnum = reader.nextDouble();
                System.out.println("Enter second num: ");
                snum = reader.nextDouble();
                answer = fnum - snum;
                System.out.println("Answer: " + answer);
                System.out.println(" ");
            }
            if (funct.equals("*"))
            {
                 System.out.println("Simple multiplication calculator");
                System.out.println("Enter first num: ");
                fnum = reader.nextDouble();
                System.out.println("Enter second num: ");
                snum = reader.nextDouble();
                answer = fnum * snum;
                System.out.println("Answer: " + answer);
                System.out.println(" ");
            }
            if (funct.equals("/"))
            {
                 System.out.println("Simple division calculator");
                System.out.println("Enter first num: ");
                fnum = reader.nextDouble();
                System.out.println("Enter second num: ");
                snum = reader.nextDouble();
                answer = fnum / snum;
                System.out.println("Answer: " + answer);
                System.out.println(" ");
            }
        }
        else if (cont.equalsIgnoreCase("n"))
        {
            break;
        }

    }
  }
}

Answer 1

In all of your function conditions inside the loop, you are using reader.nextDouble(). This will only read the number (e.g "8") input not the new line (the enter) which is entered by the user after the number. Following code for the minus(-) function will not print "Would you like to perform a calculation? (y/n) " twice as the new line is already read. Here reader.nextLine() is used instead of reader.nextDouble(). reader.nextLine() will read the whole line not only the number.

if (funct.equals("-"))
        {
            System.out.println("Simple subtraction calculator");
            System.out.println("Enter first num: ");
            fnum = Double.parseDouble( reader.nextLine());
            System.out.println("Enter second num: ");
            snum = Double.parseDouble( reader.nextLine());
            answer = fnum - snum;
            System.out.println("Answer: " + answer);
            System.out.println(" ");
        }