Ggtcf

I want to pass a list of enum classes to a method, where all of the enums implement a common interface, and have the method return one of the enum values.

Looking at Java Generics Wildcarding With Multiple Classes, it seems that

public class Main

 interface Foo 

 enum First implements Foo 
 A, B, C;
 

 enum Second implements Foo 
 X, Y, Z;
 

 interface Bar 

 enum Third implements Bar 
 M, N, P;
 
 enum Fourth implements Bar 
 A, X, Z;
 


 public static <I, T extends Enum<?> & I> 
 I enumVarArgs(Class<? extends T>... classes) 
 
 // Do stuff and return some instance of T
 return null;
 

 public static void main(String[] args) 
 Foo foo = enumVarArgs(First.class,
 Second.class);
 Bar bar = enumVarArgs(Third.class,
 Fourth.class);
 

should do what I want. However, this fails to compile under Java 10:

[ERROR] /me/test/src/main/java/test/Main.java:[17,42] error: unexpected type
 required: class
 found: type parameter I
 where I,T are type-variables:
 I extends Object declared in method <I,T>enumVarArgs(Class<? extends T>...)
 T extends Enum<?>,I declared in method <I,T>enumVarArgs(Class<? extends T>...)
[INFO] 1 error

From the error message, I am guessing that Java wants me to do something like T extends Enum<?> & Serializable, where I pass an actual interface, rather than a type parameter. However, I need the API to be general so that I remains a generic parameter.

Is there a syntax that makes this work?

If it matters, we are using Java 10.

If interfaces are your design, then you can make the two interfaces extend a marker interface:

interface FooBar

interface Foo extends FooBar 

interface Bar extends FooBar 

And you can use this marker interface:

public static <T extends Enum<?> & FooBar> 
 FooBar enumVarArgs(Class<? extends T>... classes) 
 return null;

I think what you need is:

interface OnlyImpelents<T> 
interface Foo extends OnlyImpelents<Foo> 
interface Bar extends OnlyImpelents<Bar> 

static <T extends Enum<?> & OnlyImpelents<? super T>> T enumVarArgs(T... values) 
 return values[0];

public static void main(String[] args) 
 Foo foo = enumVarArgs(First.A, Second.X);

The marker interface OnlyImpelents<T> prevents any class from implementing both Foo and Bar

The only way to get enum values from a enum class is via reflection, which doesn't need to have a static type. Personally I think this is not a good object-oriented design, using T... as args should be better