String mainwindow=driver.getWindowHandle();
driver.findElement(By.linkText("Button1")).click(); // to open the first window
for(String winHandle :driver.getWindowHandles())
{
driver.switchTo().window(winHandle);
if(driver.getTitle().equals("The sub window title you want to work on"))
{
System.out.println("You are in required window");
driver.findElement(By.linkText("Button2")).click();
break;
}
else
{
System.out.println("Title of the page after - switchingTo: " + driver.getTitle());
}
}
driver.switchTo().window(mainwindow);
driver.quit();
Let me know if it works for you.