private static String operation;

public static void main(String[] args)
{
    Scanner in = new Scanner(System.in);
    // Ask user how many numbers he/she wants to use
    System.out.print("How many numbers are you using? (3 numbers maximum) ");
    int numberFirst = in.nextInt();

    if (numberFirst > 3) {
        System.out.println("You can't do that. Please start over.");
        System.out.close();
    }
    if (numberFirst < 2) {
        System.out.println("You can't do that. Please start over.");
        System.out.close();
    }
    // Ask user to type in numbers
    System.out.println("Enter first number used: ");
    double num1 = in.nextDouble();
    System.out.println("Enter second number used: ");
    double num2 = in.nextDouble();

    if (numberFirst == 2)
        System.out.println("Press '0' and press 'enter.'");

    if (numberFirst == 3) {
        System.out.println("Enter third number used: ");
    }
    double num3 = in.nextDouble();

    // Output numbers user entered
    if (numberFirst == 2) {
        System.out.println("You are using " + num1 + " and " + num2 + ".");
    }
    if (numberFirst == 3) {
        System.out.println("You are using " + num1 + ", " + num2 + ", and " + num3 + ".");
    }
    // Put a key for the operations
    System.out.println("Addition: Enter '1'");
    System.out.println("Subtraction: Enter '2'");
    System.out.println("Multiplication: Enter '3'");
    System.out.println("Division: Enter '4'");
    if (numberFirst == 2) {
        System.out.println("Exponent: Enter '5'");
    }
    // Ask user what operation he/she wants
    System.out.println("Enter operation: ");
    operation = in.next();

    // Print the operation that user is using AND print the answer (2 numbers)
    if (numberFirst == 2) {
        if (operation.equals("1"))
            System.out.println("You are using addition." + " Your answer is " + (num1 + num2) + ".");
    }
    if (numberFirst == 2) {
        if (operation.equals("2"))
            System.out.println("You are using subtraction." + " Your answer is " + (num1 - num2) + ".");
    }
    if (numberFirst == 2) {
        if (operation.equals("3"))
            System.out.println("You are using multiplication." + " Your answer is " + (num1 * num2) + ".");
    }
    if (numberFirst == 2) {
        if (operation.equals("4"))
            System.out.println("You are using division." + " Your answer is " + (num1 / num2) + ".");
    }
    if (numberFirst == 2) {
        if (operation.equals("5"))
            System.out.println("You are using an exponent." + " Your answer is " + Math.pow(num1, num2) + ".");
    }

    // Print the operation that user is using AND print the answer (3 numbers)
    if (numberFirst == 3) {
        if (operation.equals("1"))
            System.out.println("You are using addition." + " Your answer is " + (num1 + num2 + num3) + ".");
    }
    if (numberFirst == 3) {
        if (operation.equals("2"))
            System.out.println("You are using subtraction." + " Your answer is " + (num1 - num2 - num3) + ".");
    }
    if (numberFirst == 3) {
        if (operation.equals("3"))
            System.out.println("You are using multiplication." + " Your answer is " + (num1 * num2 * num3) + ".");
    }
    if (numberFirst == 3) {
        if (operation.equals("4"))
            System.out.println("You are using division." + " Your answer is " + ((num1 / num2) / num3) + ".");