Java If statement?

public class Calculater {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        int firstNumber;

        int secondNumber;

        int thirdNumber;

        int result;

        System.out.println(" 1 - Addition ");

        System.out.println(" 2 - Subtraction ");

        System.out.println(" 3 - Division ");

        System.out.println(" 4 - Multiplication ");

        thirdNumber = scanner.nextInt();

         if (thirdNumber = 1;;

......

        Near the thirdNumber it says expression expected. Can anyone explain to me whats wrong?