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

Answer 1

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 2

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 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();
};