Handling second tab that opens on some operations in first tab in google chrome in java selenium

Question

I have test case to automate in which it will open the gmail and find a received mail using subject and in the body of the mail i have to submit comment in which it will take to the second tab in same window. And i have click on one button in the newly opened tab.

I have tried using the getWindowHandle() and getWindowHandles(). But it has only one window handler for the entire window. Using that handler i can't access the second tab.

String parentHandle = driver.getWindowHandle();
   System.out.println(parentHandle); 
   driver.findElement(By.xpath("//*[@name='comment']")).sendKeys("Comment and Approved From admin Mail");
   driver.findElement(By.xpath("//button[contains(text(),'Submit')]")).click();
   driver.switchTo().alert().accept();
   ArrayList<String> tabs = new ArrayList<String> (driver.getWindowHandles());
   System.out.println(tabs);
   tabs.remove(parentHandle);
   driver.switchTo().window(tabs.get(0));