How do I fix this Java error?

My objective is to build a basic Java calculator for 2 numbers. I got errors in else if statement.

import java.util.Scanner; public class Calculator {     public static void main(String[] args)   {     double firstNumber;     double secondNumber;     //String operator;         Scanner Calculator = new Scanner(System.in);     System.out.println("Enter your first number");     firstNumber = Calculator.nextDouble();         System.out.println("Enter your second number");     secondNumber = Calculator.nextDouble();         //char operator = Calculator.next().charAt(0);         Scanner operator = new Scanner(System.in);     System.out.println("Please enter the operator + - * / ");       String op = operator.nextLine();         if (operator == '+')       System.out.println(firstNumber + secondNumber);       else if (operator == "-")       System.out.println(firstNumber - secondNumber);     else if (op == '*')             System.out.println(firstNumber * secondNumber);     else if (operator = "/") {       System.out.println(firstNumber / secondNumber);     }       }   }