Sometimes, using a simple for loop instead of a set can prove easy to comprehend. Let's look at the ArrayList method with the same link http://popuptest.com/goodpopups.html.

The following code, when run from a main method, prints the titles of each popup:

 WebDriver driver = new ChromeDriver();
 driver.manage().window().maximize();
 driver.navigate().to("http://popuptest.com/goodpopups.html");
 try {
 } catch (InterruptedException e) {
 // TODO Auto-generated catch block
 e.printStackTrace();
 }
 driver.findElement(By.xpath("//*[text()='Good PopUp #1']")).click();
 for (String i : driver.getWindowHandles()) {
 driver.switchTo().window(i);
 System.out.println(driver.getTitle());
 }
}