How to implement an interface with an enum, where the interface extends Comparable?

Question

Consider this code:

public interface Foo extends Comparable<Foo> {}

public enum FooImpl implements Foo {}

Due to the restrictions of type erasure, I receive the following error:

java.lang.Comparable cannot be inherited with different arguments: <Foo> and <FooImpl>

I have the following requirements:

FooImpl needs to be an enum, because I need to use it as a default value in annotations.
The contract of my interface is that it needs to be comparable.

I already tried using generic bounds in the interface, but this is not supported in Java.

Try public interface Foo public enum FooImpl implements Foo, Comparable<FooImpl> {...} — Eran Zimmerman Gonen
– Eran Zimmerman Gonen, Commented Aug 23, 2011 at 12:28
The problem is that FooImpl already implements Comparable<FooImpl>. — soc
– soc, Commented Aug 23, 2011 at 12:33
And you just want to show that it does? Write it in a comment. — Eran Zimmerman Gonen
– Eran Zimmerman Gonen, Commented Aug 23, 2011 at 12:45
I need to be able to treat instances of Role as Comparable, a comment won't help here. — soc
– soc, Commented Aug 23, 2011 at 12:48
Just above public enum..., add: //Note: implements Comparable. — Eran Zimmerman Gonen
– Eran Zimmerman Gonen, Commented Aug 23, 2011 at 12:50

jvdneste · Accepted Answer · 2011-08-23 13:36:43Z

13

Enums implement Comparable, so FooImpl ends up extending Comparable twice with incompatible arguments.

The following will work:

public interface Foo<SelfType extends Foo<SelfType>> extends Comparable<SelfType> { ... }

public enum FooImpl implements Foo<FooImpl> { ... }

answered Aug 23, 2011 at 13:36

jvdneste

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1 Comment

newacct Over a year ago

just public interface Foo<SelfType> extends Comparable<SelfType> is sufficient

Community · Accepted Answer · 2017-05-23 11:46:08Z

12

Enum already implements comparable so you can't override it.

A general answer regarding why-would-an-enum-implement-an-interface.

edited May 23, 2017 at 11:46

CommunityBot

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answered Aug 23, 2011 at 12:28

NimChimpsky

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3 Comments

aioobe Over a year ago

You're avoiding the question. What if it was another interface.

NimChimpsky Over a year ago

@aioobe huh ? The question asks how to implement an interface that extends comparable

Joshua Goldberg Over a year ago

If I want to refer to my enum object via my interface: (Interface Foo {...} Enum1 implements Foo {...} Foo x1 = Enum1.FIRST;) and if I also need to use it in a context that requires a Comparable, then I have to declare that Foo extends Comparable<??something??>. Without the (admittedly clunky) typing in @jvdnese's answer, you cannot have two different Enums implement Foo and let the compiler know Foos are Comparable. (It's only a warning, so you can get away with just skipping implements Comparable). An example with the warning is TreeSet<Foo> x = new TreeSet<>();

ramsharma · Accepted Answer · 2013-11-16 13:12:57Z

1

Actually the error you will get is :

The interface Comparable cannot be implemented more than once with different arguments : Comparable<FooImpl> and Comparable<Foo>

As enum FooImpl already implementing Comparable<FooImpl> implicitly, you can not override it again as Comparable<Foo>.

answered Nov 16, 2013 at 13:12

ramsharma

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