Am I implementing a generics-based Java factory co

2020-05-26 10:18发布

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();
};