title: Why is my program entering a while loop when the condition is false?
body: <p>When I input a string operator whether it be addition(+), subtraction(-), multiplication(*), division(/) or module(%), it still enters the while loop even when I enter a valid input. I don't know what the problem could be because the while loop is working fine where I have to enter an int-value for variable num2.</p>
  
  <pre><code>import java.util.Scanner;
  
  public class PolishNotationCalc {
  
      public static void main(String[] args) {
  
          Scanner input = new Scanner(System.in);
          int num1;
          int num2;
          String operator;
  
          System.out.println("Polish notation calculator");
  
          System.out.print("Please enter an operation(+, -, *, /, %) ");
          operator = input.nextLine();
  
          while (!operator.equals("+") || !operator.equals("-") || !operator.equals("*") || !operator.equals("/") || !operator.equals("%")) {
              System.out.println("Please enter a valid operation ");
              operator = input.nextLine();
              if (operator.equals("+") || operator.equals("-") || operator.equals("*") || operator.equals("/") || operator.equals("%"))
                  break;
          }
  
          System.out.print("");
          System.out.print("Please enter the first number ");
          num1 = input.nextInt();
  
          System.out.print("Please enter the second number ");
          num2 = input.nextInt();
  
          while (num2 == 0 &amp;&amp; operator.equals("/")) {
              System.out.println("Please pick a non zero number: ");
              num2 = input.nextInt();
          }
          while (num2 == 0 &amp;&amp; operator.equals("%")) {
              System.out.println("Please pick a non zero number: ");
              num2 = input.nextInt();
  
          }
  
          if (operator.equals("+"))
              System.out.println(num1 + " + " + num2 + " = " + (num1 + num2));
  
          else if (operator.equals("-"))
              System.out.println(num1 + " - " + num2 + " = " + (num1 - num2));
  
          else if (operator.equals("*"))
              System.out.println(num1 + " * " + +num2 + " = " + (num1 * num2));
  
          else if (operator.equals("/"))
              System.out.println(num1 + " / " + num2 + " = " + (num1 / num2));
  
          else if (operator.equals("%"))
              System.out.println(num1 + " % " + num2 + " = " + (num1 % num2));
  
      }
  
  }
  </code></pre>