Question

I’m look for good alternatives to invoking a specific interface from a generic framework. I’ll exemplify with code. Look to the question part, the example code is primarily included for thoroughness, and for putting the example into a real scenario.

Example

Assume we want to build a report based on a list of components. Say we have two specific component types:

public interface Component { ... }
public class PDFComponents extends Component { ... }
public class WordComponents extends Component { ... }

Each component has a ReportBuilder implementation, e.g.,

public interface ReportBuilder { ... }
public class PDFReportBuilder extends ReportBuilder { ... }
public class WordReportBuilder extends ReportBuilder { ... }

Which builds specific report implementations

public interface Report { ... }
public class PDFReport extends ReportBuilder { ... }
public class WordReport extends ReportBuilder { ... }

Finally we have the service which locates components and produces a report out of the components.

public class ReportService {
    ReportComponentRepository repo;
    List<ReportBuilder> builders;

    public <T extends Report> T getReport(Class<T> reportType) {
        // Get report components. E.g., this might return List<PDFComponent>
        List<Component> reportComponents = repo.getReportComponents(id);

        // Build report from components using one of the registered builders
        for (ReportBuilder builder : builders) {
            if (builder.buildFor(reportType) {
                return builder.buildReport(report);
            }
        }
    }
}

Example using the service

List<PDFReport> reports = new ReportService().getReport(PDFReport.class);

Question

Now to the question. How can I design a generic ReportBuilder interface which allows typesafety to it’s implementations?

E.g., choosing the interface:

public Report buildReport(List<? extends Component> components);

would cause ugliness in it’s implementations:

public class PDFReportBuilder implements ReportBuilder {

    @Override
    public Report buildReport(List<? extends Component> components) {
         PDFReport report;

         for (Component component : components) {
            if (component instanceOf PDFComponent) {
                // assemble report ... 
                report.includeComponent(component);
            }
        }

        return report;
    }
}

when we really desire the interface for PDFReportBuilder to be e.g.,

 public Report buildReport(List<PDFComponent> component) { ... }

Answer 1

It works if you make the type of Component into a type variable for ReportBuilder:

public interface ReportBuilder<T extends Component> {
    public Report buildReport(List<T> components);
}

public class PDFReportBuilder implements ReportBuilder<PDFComponent> {
    public Report buildReport(List<PDFComponent> components);
}

You’ll have to evaluate whether you really want a type variable in ReportBuilder though. It’s not always the correct choice. Additionally, if you also want PDFReportBuilder.buildReport to have a return type that is PDFReport, then you need to have that as a type variable as well (i.e., public interface ReportBuilder<T extends Component, S extends Report>).