Need to perform validation testing for field

Hello ,

This is my code for webdriver where is assert for Alertbox , but i have a field where instead of alertbox validation text is showned when clicked on button,.This is code i get for asserting text validation

Code:

   assertEquals("Special Characters are not allowed.",
driver.findElement(By.cssSelector("span.field-validation-error >
span")).getText());

How to automate that scenario

Code:

import java.util.concurrent.TimeUnit;

import org.openqa.selenium.By;
import org.openqa.selenium.firefox.FirefoxDriver;


public class FirstName{
 
 public static void main(String[] args) throws Exception {
    String[] invalidChars = {"#", "!", "$", "@", "%", "^", "&"};
    String name = "acbcdefghijklmnopqrstuvwxyzab";
    FirefoxDriver d = new FirefoxDriver();
    d.manage().timeouts().implicitlyWait(30, TimeUnit.SECONDS);
    d.get("websiteurl gors here/Register");
    for (String invalid : invalidChars) {
        d.findElement(By.id("FirstName")).clear();
        d.findElement(By.id("FirstName")).sendKeys(name + invalid);
        d.findElement(By.id("CreateAccount")).click();
                String alertMessage = d.switchTo().alert().getText();
        System.out.println(invalid);
        if (alertMessage.equals("Special Characters are not allowed."))
        {
            System.out.println("Error displayed: First name Should not contain Special Characters");
            d.switchTo().alert().dismiss();
        } else {
            System.out.println("Accepted");
        }
    }
    
    
    d.quit();
 }

}