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

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");

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);