selenium code for popup window and get text alert message

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.interactions.Actions;

public class JquerySite
{

public static void main(String[] args) {
WebDriver driver=new FirefoxDriver();
driver.get("http://jqueryui.com/droppable/");
driver.manage().window().maximize();

driver.switchTo().frame(driver.findElement(By.xpath("//iframe[@src='/resources/demos/droppable/default.html']")));

WebElement source=driver.findElement(By.xpath(".//*[@id='draggable']"));

//WebElement target=driver.findElement(By.xpath(".//*[@id='droppable']"));

Actions dd=new Actions(driver);

dd.dragAndDropBy(source, 150, 30).build().perform();
//dd.dragAndDropBy(target, 20, 250).build().perform();
}

}