i was having some issues with windowhandle and tried this one. this one works good for me.
String parentWindowHandler = driver.getWindowHandle();
String subWindowHandler = null;
Set<String> handles = driver.getWindowHandles();
Iterator<String> iterator = handles.iterator();
while (iterator.hasNext()){
subWindowHandler = iterator.next();
driver.switchTo().window(subWindowHandler);
System.out.println(subWindowHandler);
}
driver.switchTo().window(parentWindowHandler);