How to write code if not an element exists in selenium [JAVA]

This a login test scenario. There are two user. They are valid and invalid users. Pop-up OK click is working in testing.

<---- PROBLEM ----->

If ----> (if not an element exists) ---- print("successfuly login"),

else ----> (if an element exists) ----- pop-up ok click. Login with other users.

I can't do this no such element case.

ERROR

 Exception in thread "main" org.openqa.selenium.NoSuchElementException: no such element: Unable to locate element: {"method":"xpath","selector":"html/body/div/div/div[3]/button[1]"}
  (Session info: chrome=79.0.3945.88)

CODE

    WebElement temp444 = driver.findElement(By.xpath("html/body/div/div/div[3]/button[1]"));
    System.out.println(temp444.getSize());
    if(temp.getSize()!=null){

         temp.click(); // ----> It's working!

    }
    else {

    System.out.println("good working"); // ----> It isn't working.

    }