Am I implementing a generics-based Java factory correctly?

问题

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);
}

回答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();
    };
}

回答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);
  }
}

回答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();
};

来源：https://stackoverflow.com/questions/6191710/am-i-implementing-a-generics-based-java-factory-correctly

标签

java

generics

factory-pattern