If statement used to check entered string for math operators and throws error of "java.lang.NumberFormatException"

My program asks for 3 inputs, num1, num2 and math operator in this order. All of those are strings. My If statement is checking if the input for num1 or num2 was not a math operator.

import java.text.DecimalFormat;

import java.util.*; public class assignment {

public static void main (String[] args) {

    Scanner scan = new Scanner (System.in);
    int sum, HighRes, LowRes, Agg, Ave, Amount, Wrong, Right;
    String operator, another = "y", num1, num2;
    HighRes = 0;
    LowRes = 0;
    Agg = 0;
    Amount = 0;
    DecimalFormat df = new DecimalFormat("#.00"); 
    Wrong = 0;
    Right = 0;


    while (another.equalsIgnoreCase("y"))
            {
            System.out.println("Enter the first number, second number and the operator (+, -, * or /)");
            num1 = scan.next();
            num2 = scan.next();       
            operator =scan.next();
            System.out.println( "Expression is " + num1 + " " +num2 + " " +operator);

            if (num1 == "-"  || num1 == "+" || num1 == "*" || num1 == "/" || num2 == "+" || num2 == "+" || num2 == "+" || num2 == "+"){
                System.out.println("Invalid expression");
                System.out.print ("Test another expression (Type Y if yes and leave empty if no?) ");
                another = scan.nextLine();
                Wrong++;
            }
            else      {    
                int num3 = Integer.parseInt(num1);
                int num4 = Integer.parseInt(num2);

I'm not advanced in programming so, I am sorry if my code makes you cringe. For me it seems that if the entered string for num1 and num2 is either -,+,* or /, it should print "Invalid expression" but what it does is giving me "java.lang.NumberFormatException" error

Line 35 and 36 are when I am converting the string to integer, and to me that should work since it's in the else statement and the If does not trigger