類型安全實現的Java通用接口

Question

我正在尋找從通用框架調用特定接口的好替代方案。我將用代碼來舉例說明。請看問題部分，示例代碼主要是爲了徹底性，並將該示例放入真實場景中。

示例

假設我們要基於組件列表構建報告。假設我們有兩個特定組件類型：

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

每個組件都有一個報表製作工具實現，例如，

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

它建立特定的報表實現

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

最後我們有定位服務組件並從組件生成報告。

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); 
      } 
     } 
    } 
} 

例如，使用服務

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

問題

我們的問題。我如何設計一個通用的ReportBuilder接口，允許類型安全實現？

例如，選擇界面：

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

會導致醜陋，在它的實現：

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; 
    } 
} 

當我們真正想要的界面爲PDFReportBuilder是如

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

Answer 1

它適用於將組件的類型轉換爲ReportBuilder的類型變量：

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

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

您必須評估您是否確實想要ReportBuilder中的類型變量。這並不總是正確的選擇。此外，如果您還希望PDFReportBuilder.buildReport的返回類型爲PDFReport，那麼您還需要將其作爲類型變量（即public interface ReportBuilder<T extends Component, S extends Report>）。

Answer 2

在我看來，你似乎設置了一個凌亂的實現，有三個並行繼承層次結構。請問，爲什麼你不能合併Component和ReportBuilder的共享行爲？實際上，通過強制服務調用者知道他們想要的報告的子類，您會失去對組件的任何抽象。

我建議通過減少或消除參數buildReport（）

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

    public <T extends Report> T getReport(Class<T> reportType) { 

     // Build report from components using one of the registered builders 
     for (ReportBuilder builder : builders) { 
      if (builder.buildFor(reportType) { 
       //don't pass components - if there's a requirement 
       //for a strongly typed subclass of Component, just 
       //let the Report instance figure it out. 
       return builder.buildReport(); 
      } 
     } 
    } 
} 


//example use 
public class PDFReportBuilder implements ReportBuilder { 

    ComponentSource componentSource; 

    @Override 
    public Report buildReport() { 
     PDFReport report; 

     for (PDFComponent component : componentSource.getPDFComponents()) { 
      // assemble report ... 
      report.includeComponent(component); 
      // no instanceof operations! 
     } 

     return report; 
    } 
}