Question

The code below is a simplified version of the pattern my project is using. The standard pattern we use is to have a Writer for each object type. For the subtypes of one abstract type (in this example Animal), I’d like an enum to serve as a lookup for the correct writer.

abstract class Writer<T> {
    abstract void write(T value);
}

abstract class Animal {
    abstract AnimalType getType();
}

class Cat extends Animal {
    AnimalType getType() { return AnimalType.CAT; }
}

class CatWriter extends Writer<Cat> {
    void write(Cat value) { }
}

// The AnimalType stores a reference to the correct writer for the Animal subclass
enum AnimalType {
    CAT(new CatWriter());

    Writer<? extends Animal> writer;
    Writer writerThatWorksWithWarning;
    Writer<Animal> writerThatWorksButCantBeAssigned;

    AnimalType(Writer<? extends Animal> writer) {
        this.writerThatWorksWithWarning = writer;
        this.writer = writer;

        // ERROR: Incompatible Types
        this.writerThatWorksButCantBeAssigned = writer;

    }
}

Sample use case:

class Test {
    public static void main(String... args) {
        Animal value = new Cat();

// ERROR: write (capture<? extends Animal) in Writer cannot be applied to (Animal)
        value.getType().writer.write(value);

// WARNING: Unchecked call
        value.getType().writerThatWorksWithWarning.write(value);

// This line works fine here - but can't be assigned above
        value.getType().writerThatWorksButCantBeAssigned.write(value);
    }
}

I think that my problem is similar to the problem in this question: Java Generics with wildcard, however I can’t tell how to solve it.

I’ve put the inline errors and warnings I get in the comments.

Any ideas?

Answer 1

Try this instead,

enum AnimalType {
    CAT(new CatWriter());

    private Writer<? extends Animal> writer;

    AnimalType(Writer<? extends Animal> writer) {
        this.writer = writer;
    }

    public Writer<Animal> getWriter() {
        return (Writer<Animal>)writer;
    }
}

Moreover, I am not sure what are you up to. But I believe that Visitor pattern will come handy in this case.

Problem with the above solution, the code below will break the thing.

    Animal cat = new Cat();
    Animal dog = new Dog();

    cat.getType().getWriter().write(cat);

    // java.lang.ClassCastException in the write() method's argument
    cat.getType().getWriter().write(dog);