Cannot resolve method error when trying to reproduce the code - symbols

Question

I know it's been a while since this issue was resolved ( Simple calculator in java - using boolean to ask if user wants to continue ), but I wanted to recreate this, and when I tried to run it, I got error for all mathematical operators (add, minus, multiply or divide). Error says: "Cannot resolve method 'add'/'minus'/'multiply'/'divide'"
Maybe it would be easier if I add the actual code:
import javax.swing.JOptionPane;
public class Calculator {
public static void main(String[] args) {
double numberOne;
double numberTwo;
double result;
String number1;
String number2;
String input;
boolean useCalculator = false;
while (!useCalculator) {
number1 = JOptionPane.showInputDialog(null, "This is a calculator\nEnter first number");
numberOne = Double.parseDouble(number1);
input = JOptionPane.showInputDialog(null, "Choose from the following options:\n+\n-\n*\n/");
while (input == null || !(input.equals("+") || input.equals("*") || input.equals("/") || input.equals("-")))
input = JOptionPane.showInputDialog(null, "Thank you! What would you like to do?\nPlease choose from the following options:\n+\n-\n*\n/");
{
input = JOptionPane.showInputDialog(null, "The operator" + input + "is invalid. Please choose a correct one");
}
number2 = JOptionPane.showInputDialog(null, "Enter second number");
numberTwo = Double.parseDouble(number2);
if (input != null && input.equals("+")) {
add(numberOne, numberTwo);
result = add(numberOne, numberTwo);
JOptionPane.showMessageDialog(null, "The result of " + numberOne + " " + input + " " + numberTwo + " is: " + result);
} else if (input != null && input.equals("-")) {
minus(numberOne, numberTwo);
result = minus(numberOne, numberTwo);
JOptionPane.showMessageDialog(null, "The result of " + numberOne + " " + input + " " + numberTwo + " is: " + result);
} else if (input != null && input.equals("*")) {
multiply(numberOne, numberTwo);
result = multiply(numberOne, numberTwo);
JOptionPane.showMessageDialog(null, "The result of " + numberOne + " " + input + " " + numberTwo + " is: " + result);
} else if (input != null && input.equals("/")) {
divide(numberOne, numberTwo);
result = divide(numberOne, numberTwo);
JOptionPane.showMessageDialog(null, "The result of " + numberOne + " " + input + " " + numberTwo + " is: " + result);
}
}
System.out.println("End of program.");
}
}
Any thoughts on how to fix this and run it without those errors?
Thanks in advance :)

Answer 1

It looks like GUI was running in the background, so when I minimized window, I was able to see the calculator and to run it normally.
Message that was displayed in console was reminder about old JDK, so everything is fine now.