One Script, Every Browser

A loop that calls methods on a Browser never mentions Chrome, Firefox, or Edge by name — because the method that runs is chosen from the actual object at the moment of the call, not the declared type.

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Here is why any of this matters. The Browser contract is signed by three classes, and the test code below knows about none of them:

String[] wanted = {"chrome", "firefox", "edge"};
for (String name : wanted) {
    Browser b = create(name);
    b.open("https://uat.shopcart.internal/login");
    b.close();
}

[Chrome ] navigating to https://uat.shopcart.internal/login
[Chrome ] closed
[Firefox] navigating to https://uat.shopcart.internal/login
[Firefox] closed
[Edge   ] navigating to https://uat.shopcart.internal/login
[Edge   ] closed

Look at what the loop body does not contain. No if for the browser type. No mention of Chrome, Firefox, or Edge. It calls open and close on something that is a Browser, and the right code runs every time.

That is runtime polymorphism: the method is chosen from the actual object at the moment of the call, not from the declared type at compile time. It is the same mechanism you saw with BaseTest t = new ApiTest() in Chapter 11, now doing the job it was built for. This is precisely how cross-browser automation works, and it is the answer to "why is WebDriver an interface?" Because a test written against the interface is written against every browser at once. Add a fourth browser tomorrow, and the loop above does not change.

The declared type and the actual object are genuinely different things, and you can see both:

Browser b = new FirefoxBrowser();
System.out.println("Declared type : Browser");
System.out.println("Actual object : " + b.getClass().getSimpleName());

[Firefox] navigating to https://uat.shopcart.internal
Declared type : Browser
Actual object : FirefoxBrowser

Storing a FirefoxBrowser in a Browser variable is called upcasting, and Java does it automatically because a FirefoxBrowser genuinely is a Browser. The variable limits what you may call — only what the contract lists — while the object decides what actually runs.

Given Browser b = new FirefoxBrowser(); — which class's open method runs? Can you call a method that exists only on FirefoxBrowser and not on Browser? Why can you not write new Browser()?

Answer — FirefoxBrowser's, because the object decides. No — the declared type limits what you may call. And an interface has no method bodies, so there is nothing to run.

This exact pattern — declare the general capability, create one specific kind — is not new to this chapter. You've been writing it since Chapter 5, on faith.