2nd Example :-
 

String parentwindow = driver.getWindowHandle(); // get the current window handle

driver.findElement(By.xpath("//*[@id='someXpath']")).click(); // click some link that opens a new window

for (String winHandle : driver.getWindowHandles()) {

    driver.switchTo().window(winHandle); // switch focus of WebDriver to the next found window handle (that's your newly opened window)

}

//code to do something on new window

driver.close(); // close newly opened window when done with it

driver.switchTo().window(parentwindow); // switch back to the original window

3rd Example :-

// Get and store both window handles in array

  Set<String> Allwindows = driver.getWindowHandles();

// Extract parent and child window handle from all window handles

  String window1 = (String) Allwindows.toArray()[0];

  System.out.print("window1 handle code = "+Allwindows.toArray()[0]);

  String window2 = (String) Allwindows.toArray()[1];

  System.out.print("newwindow2 handle code = "+Allwindows.toArray()[1]);

 
  //Switch to window2(child window) and performing actions on it.

  driver.switchTo().window(window2);