Implement interface with enum value as generic parameter

Question

Let's assume I have some enum:

enum Resource { FILE, URL, STREAM }

Let's assume I have some Reader interface:

interface Reader<R extends Resource>

Is it possible to create different implementations of it using Enum Members? Like that:

class FileReader implements Reader<Resource.FILE>

My ide highlights this text with red (which is not a big surprise).

Short answer, no, long answer, making a method in your enumerated and override in each enumerated value might be a solution — user180100
– user180100, Commented Jan 20, 2014 at 7:47

Alexander Tokarev · Accepted Answer · 2014-01-20 07:49:43Z

2

Enum members are instances of enum type, not subclasses. Moreover, enums can't be subclassed. Thus, you can't compile your code.

I suggest you to write this way:

interface Reader {
    Resource getResource()
    ....
}

class FileReader implements Reader {
    @Override
    public Resource getResource() {return Resource.FILE;}
    ...
}

answered Jan 20, 2014 at 7:49

Alexander Tokarev

2,7632 gold badges21 silver badges21 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

yshavit Over a year ago

Enum members can actually be subclasses of the enum type, if they declare a class body (ie, if the enum name is followed by curly braces). But this extended class is an anonymous class, and as such has no name and can't be referenced in a generic declaration. See JLS 8.9.1.

yshavit · Accepted Answer · 2014-01-20 08:08:40Z

As Alexander Tokarev mentioned, you can't do this. An alternative to his approach is to roll your own enum-like classes by extending nested classes from a base class that has a private constructor:

public class Resource {
    private Resource() {}

    // some methods, abstract methods, etc...

    public static class File extends Resource {
      private File() {}

      // etc...
    }

    private static final File FILE = new File();

    // other subclasses of Resource
}

Now you can use generics as usual:

interface Reader<R extends Resource> {...}
class FileReader implements Reader<Resource.File> {...}

This gives you many of the advantages that enums have -- type safety, a closed set of values, etc -- while explicitly setting up a class hierarchy that you can use with generics. The major cost is in brevity of code, though you also lose the ability to use the values in switch statements, as well as some other, more esoteric benefits of enums.

Collectives™ on Stack Overflow

Implement interface with enum value as generic parameter

2 Answers 2

1 Comment

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Related