Am I implementing a generics-based Java factory correctly?

Question

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

In other words, is there a way I can restrict the createDocument method to only accept subclasses of 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);
  }
  

Answer 1

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

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

Answer 2

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.

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

[update:] Code sample:

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

Answer 3

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

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