我是否正确实现了基于泛型的 Java 工厂？

Am I implementing a generics-based Java factory correctly?

提问by Andrew

I don't believe I am implementing the factory pattern correctly because the Applicationclass' createDocumentmethod accepts any class type, not just subclasses of Document.

我不相信我正确地实现了工厂模式，因为Application类的createDocument方法接受任何类类型，而不仅仅是Document.

In other words, is there a way I can restrict the createDocumentmethod to only accept subclasses of Document?

换句话说，有没有办法限制createDocument方法只接受 的子类Document？

• Document.java

  package com.example.factory;
  
  public abstract class Document {
      public Document() {
          System.out.println("New Document instance created: " + this.toString());
      }
  }
  

• DrawingDocument.java

  package com.example.factory
  
  public class DrawingDocument extends Document {
      public DrawingDocument() {
          System.out.println("New DrawingDocument instance created: " this.toString());
      }
  }
  

• Application.java

  package com.example.factory;
  
  public class Application {
      public <T> T createDocument(Class<T> documentClass) {
          try {
              return documentClass.newInstance();
          } catch (InstantiationException e) {
              throw new IllegalArgumentException(e);
          } catch (IllegalAccessException e) {
              throw new IllegalArgumentException(e);
          }
      };
  }
  

• Main.java

  package com.example.factory;
  
  public static void main(String[] args) {
      Application application = new Application();
      application.createDocument(DrawingDocument.class);
  }
  

• 文档.java

  package com.example.factory;
  
  public abstract class Document {
      public Document() {
          System.out.println("New Document instance created: " + this.toString());
      }
  }
  

• 绘图文档.java

  package com.example.factory
  
  public class DrawingDocument extends Document {
      public DrawingDocument() {
          System.out.println("New DrawingDocument instance created: " this.toString());
      }
  }
  

• 应用程序.java

  package com.example.factory;
  
  public class Application {
      public <T> T createDocument(Class<T> documentClass) {
          try {
              return documentClass.newInstance();
          } catch (InstantiationException e) {
              throw new IllegalArgumentException(e);
          } catch (IllegalAccessException e) {
              throw new IllegalArgumentException(e);
          }
      };
  }
  

• 主程序

  package com.example.factory;
  
  public static void main(String[] args) {
      Application application = new Application();
      application.createDocument(DrawingDocument.class);
  }
  

回答by Yet Another Geek

You should boundyour generic so that it is only using T's that inherit Document. example:

您应该绑定您的泛型，以便它仅使用继承 Document 的 T。例子：

public class Application {
    //Add extends Document after T
    public static <T extends Document> T createDocument(Class<T> documentClass) throws InstantiationException, IllegalAccessException {
        return documentClass.newInstance();
    };
}

回答by Roland Illig

The code looks good. In a real implementation the factory method should not be declared to throw any of the reflection-related exceptions. And you will probably have some different code anyway to create the document.

代码看起来不错。在实际实现中，不应将工厂方法声明为抛出任何与反射相关的异常。无论如何，您可能会有一些不同的代码来创建文档。

The faxtory method should take a Class<? extends Document>as its parameter, so that one cannot ask it to create a String, for example.

例如，faxtory 方法应该将 aClass<? extends Document>作为其参数，这样就不能要求它创建一个String。

[update:] Code sample:

[更新：] 代码示例：

public Document createDocument(Class<? extends Document> clazz) {
  try {
    return clazz.newInstance();
  } catch (InstantiationException e) {
    throw new IllegalArgumentException(e);
  }
}

回答by Grooveek

Where is the restriction to Document type in the factory ? Try

工厂对文件类型的限制在哪里？尝试

public <T extends Document> T createDocument(Class<T> documentClass) throws InstantiationException, IllegalAccessException {
    return documentClass.newInstance();
};