A small demo of Selenium using java 8 to show the use case of multi-windows pop ups and accessing the data within them.

Selenim_Demo.java

/**
 * 
 */
package com.selenium.demo;

import static org.junit.jupiter.api.Assertions.*;

import java.util.Iterator;
import java.util.Set;

import org.junit.jupiter.api.Test;
import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;



/**
 * @author Shubham
 *
 */
class Selenim_Demo {

	@Test
	void test() {
		String ChromDriver = "D:\\Selenium-Automation\\chromedriver.exe";
		System.setProperty("webdriver.chrome.driver", ChromDriver);
		WebDriver browser = new ChromeDriver();
		// Navigate Browser to MT URL
		String MT_URL = "D://Selenium-Automation//index.html";
		browser.get(MT_URL);
		//Get the Main Window Id to maintain the to and fro between the windows.
		String MainWindow=browser.getWindowHandle();
		System.out.println("MainWindow id is >"+MainWindow);
		//Click on the p tag
		browser.findElement(By.xpath("//*[@id=\"p_click\"]")).click();
		waitForPageFullyLoaded(2, 1,browser);
		//Now Iterating over browser to get the child windows:
		// To handle all new opened window.				
        Set<String> s1=browser.getWindowHandles();		
        Iterator<String> i1=s1.iterator();
        System.out.println("Before While Loop");
        while(i1.hasNext())			
        {		
            String ChildWindow=i1.next();		
            System.out.println("Inside While Loop");
            if(!MainWindow.equalsIgnoreCase(ChildWindow))			
            {    		
            		System.out.println("Inside Child Window");
                    // Switching to Child window
            		browser.switchTo().window(ChildWindow);
            		//Entering the value
            		System.out.println("Setting Value");
            		browser.findElement(By.xpath("//*[@id=\"p_text\"]"))
                    .sendKeys("Shubham");    
            		//Submitting the value to MainWindow
            		browser.findElement(By.xpath("//*[@id=\"p_click\"]")).click();
            }
        }
        //Back to Main Windows
        
	}
	/**
	 * Soft wait for loading of all elements in page. This will check for count
	 * of loaded elements in page in every specified timeSlice interval till
	 * maxTimeout is achieved. Once count remains same for two consecutive
	 * checks, it assumes all elements loaded and returns true.
	 * @author Shubham
	 * @param maxtimeout
	 *            Max time to wait in seconds
	 * @param timeSlice
	 *            slice of time interval to check for elements count in seconds.
	 * @return
	 */
	
	public boolean waitForPageFullyLoaded(int maxtimeout, int timeSlice,WebDriver  browser) {
		int previous;
		int current = 0;		
		do {
			previous = current;
			sleep(timeSlice);
			maxtimeout -= timeSlice;
			current = browser.findElements(By.xpath("//*")).size();
		} while (current != previous && maxtimeout > 0);
		if (maxtimeout > 0) {
			return true;
		}
		
		//Log.error("Page didnt load completely in "+maxtimeout+" seconds.");
		return false;
	}
	/**
	 * Causes the currently executing thread to sleep (temporarily cease
	 * execution) for the specified number of seconds, subject to the precision
	 * and accuracy of system timers and schedulers. The thread does not lose
	 * ownership of any monitors.
	 * 
	 * @param timeout
	 *            the length of time to sleep in seconds
	 * @author Shubham
	 */
	public void sleep(int timeout) {
		try {
			Thread.sleep(timeout * 1000);
		} catch (Exception e) {
			//Log.error("Interrupted Exception Occurred in sleep.");
			//Log.debug(e.getMessage());
		}
	}

}