How to type some text in hidden field in Selenium WebDriver using Java

WebDriver driver=new FirefoxDriver();
driver.get("http://localhost/login.do");
driver.manage().window().maximize();
RemoteWebDriver r=(RemoteWebDriver) driver;
String s1="document.getElementById('username').value='admin'";
r.executeScript(s1);

First of all you have to change the value of type attribute as text from hidden. The following code using javascript would work for that:

jse.executeScript("document.getElementsByName('body')[0].setAttribute('type', 'text');");

Now, you are able to type on that text by using WebDriver. So, the overall code for typing in a hidden field with WebDriver using Java and Javascript as follows:

WebDriver driver = new FirefoxDriver();
JavascriptExecutor jse = (JavascriptExecutor)driver;
jse.executeScript("document.getElementsByName('body')[0].setAttribute('type', 'text');");
driver.findElement(By.xpath("//input[@name='body']")).clear();
driver.findElement(By.xpath("//input[@name='body']")).sendKeys("Ripon: body text");