How to switch control to pop-up window?

If you want to do any operations in pop-up window you need to switch the control to pop-up window then do all your operations in that and finally close the pop-up window and again select the default (main ) window.
Here is WebDriver logic to select Pop-up window

1. Pop-up window has name/id

     driver.switchTo().window("<window name>");

2. Pop-up window doesn't have name / you don't want to hard code the window name then go for below logic.

Before opening pop-up get the main window handle.
 
             String mainWindowHandle=driver.getWindowHandle();

Open the pop-up (click on element which causes open a new window)
 
             webElement.click();

Try to get all available open window handles with below command. (The below command returns all window handles as Set)

            Set s = driver.getWindowHandles();

From that above set try to get newly opened window and switch the control to that (pop-up window handle), as we already know the mainWindowHandle.  

               Set s = driver.getWindowHandles();

              Iterator ite = s.iterator();
             while(ite.hasNext())
               {
               String popupHandle=ite.next().toString();
              if(!popupHandle.contains(mainWindowHandle))
               {
                   driver.switchTo().window(popupHandle);
              }
           }