How to perform drag and drop operation using webdriver

gistfile1.java

import java.util.concurrent.TimeUnit;
import org.openqa.selenium.*;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.interactions.Actions;


public class DragAndDropExample 
{
	public static void main(String[] args) throws InterruptedException 
	{
		WebDriver driver=new FirefoxDriver();
		driver.get("http://test.cita.illinois.edu/aria/draganddrop/draganddrop3.php");
		driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
		driver.manage().window().maximize();
		
		driver.findElement(By.id("startButton")).click();
		Thread.sleep(3000);
		
		WebElement target=driver.findElement(By.xpath("//td[@id='cell1x1']"));
		WebElement source =driver.findElement(By.xpath("//div[@id='ex']/img"));
		
		Actions action=new Actions(driver);
//		action.clickAndHold(source).moveToElement(target).release(target).perform();
//		action.clickAndHold(source).moveToElement(target).release(target).build().perform();
//		action.clickAndHold(source).moveToElement(target).release().perform();
		action.dragAndDrop(source, target).perform();
	}

}