Interface versus implementation

I've seen a few people posting Java code for Selenium automation where they have things like:

FirefoxDriver driver = new FirefoxDriver();

If you look at the code for Selenium you will see they have an interface called WebDriver. FirefoxDriver, InternetExplorerDriver, HtmlUnitDriver, RemoteWebDriver all implement WebDriver.

If I'm writing a test suite, I ultimately want to run the suite on Firefox, Internet Explorer, Chrome, etc. but if I implement my framework using FirefoxDriver, I have to edit the code to make it work on InternetExplorerDriver. The proper solution is to write your code using:

WebDriver driver = new FirefoxDriver();

You might argue that I need to change my code still. Okay, so you would implement it using:

WebDriver driver;
    if(browserType.equals("firefox")) {
        driver = new FirefoxDriver();
    } else if(browserType.equals("ie")) {
        driver = new InternetExplorerDriver();
    } else {
        driver = new RemoteWebDriver();
    }

Now all subsequent code will use driver as if it was merely a WebDriver. However, if you look at RemoteWebDriver you will see that it also implements JavascriptExecutor. So if I wanted to use:

driver.executeScript(script, args);

I'll get an error because WebDriver does not support executeScript. My driver is REALLY a RemoteWebDriver and does support executeScript. So how do I access that functionality? Quite simple:

((RemoteWebDriver)driver).executeScript(script, args);

Even better would be:

if(driver instanceof RemoteWebDriver) {
        ((RemoteWebDriver)driver).executeScript(script, args);
    } else if(driver instanceof FirefoxDriver) {
        ((FirefoxDriver)driver).executeScript(script, args);
    } else if(driver instanceof InternetExplorerDriver) {
        ((InternetExplorerDriver)driver).executeScript(script, args);
    }

Now if you run the test suite against a browser that does not support a particular feature, it will skip that part of the code automatically.

And that is proper use if polymorphism.