Java/Selenium：方法中尚未定义 class 的实例

Question

In Java, it is possible to define a return value T, extending an abstract base class - and in the return method, to cast T into an instance of a specific non-abstract class.

Example: My website is supposed to show a registration page in European countries, and a terms+conditions page elsewhere. I have a test method providing this:

public <T extends StartBasePage> T openStartPage(boolean isInEurope) {
        if (isInEurope) {
            return (T) pageProvider.createPageAsserted(StartRegistrationPage.class);
        } else {
            return (T) pageProvider.createPageAsserted(StartTermsPage.class);
        }
    }

But is it possible to use T in the method already as an "instance" of abstract StartBasePage and then use methods from that abstract class to define what exactly is supposed to be cast into?

Imagine StartBasePage has a method extracting the country ISO-2 code in which the starting page was opened; and there is a boolean method to check if a country code is from a European country:

public <T extends StartBasePage> T openStartPage() {
        if (T.extractCountryFromStartPage().isInEurope()) {
            return (T) pageProvider.createPageAsserted(StartRegistrationPage.class);
        } else {
            return (T) pageProvider.createPageAsserted(StartTermsPage.class);
        }
    }

It is not possible as written, but is there an adaption making this possible?

Answer 1

The Null Object Pattern encapsulates behavioral differences like this while keeping the overall interaction the same. The openStartPage method should return a TermsAndConditionsPage whether the user is in Europe or elsewhere. There would be a single sub class of TermsAndConditionsPage, named something like NullTermsAndConditionsPage or MissingTermsAndConditionsPage. Calling the acceptTermsAndConditions() method should accept the terms and conditions, or simply do nothing. Then both cases should return the StartRegistrationPage.

public class TermsAndConditionsPage {
    private final WebDriver driver;

    public TermsAndConditionsPage(WebDriver driver) {
        this.driver = driver;
    }

    public StartRegistrationPage acceptTermsAndConditions() {
        // Click the "Accept" button and wait

        return new StartRegistrationPage(driver);
    }
}

public class NullTermsAndConditionsPage extends TermsAndConditionsPage {
    @override
     public StartRegistrationPage acceptTermsAndConditions() {
        return new StartRegistrationPage(driver);
    } 
}

Then modify your openStartPage method to detect if the user is in Europe and conditionally return either a new TermsAndConditionsPage or a NullTermsAndConditionsPage:

public TermsAndConditionsPage openStartPage() {
    if (isInEurope()) {
        return new NullTermsAndConditionsPage(driver);
    } else {
        return new TermsAndConditionsPage(driver);
    }
}

Calling and interacting with the openStartPage method is normalized between countries that require a ToS and those that do not:

StartRegistrationPage = openStartPage().acceptTermsAndConditions();

When in Europe, the worse thing that happens is the NullTermsAndConditionsPage.acceptTermsAndConditions() method gets called and does absolutely nothing except return the StartRegistrationPage object, since in those flows that is the page the user lands on.