为什么在使用通配符调用Enum.valueOf（）时出现绑定不匹配错误

Question

I am trying to call the Enum<E> class's static method valueOf() but I received a compile error. Please look at the code snippet below.

public void hello(Class<? extends Enum<?>> q){  
    Object o= Enum.valueOf(q,"hello");      
}

IntelliJ IDEA complies the following code just fine, but Eclipse gives a compile error:

Answer 1

I don't know IntelliJ's workings, but I can tell you the reason why it doesn't compile.

The method defined in the class Enum<T extends Enum<T>>

public static <T extends Enum<T>> T valueOf(Class<T> enumType, String name)

has a first argument that is the class of a type T which is defined by T extends Enum<T> . That is, it extends an enumeration type with the same generic type as itself.

When you call the method with the first argument as

Class<? extends Enum<?>>

you are not specifying that the two ? are in fact the same type, as required. This throws the mismatch error.

What you can do with your method is turn it generic:

public static <E extends Enum<?>> void hello(Class<E> q)

but as you can imagine, this will still not compile because ? is not necessarily E . So you need to spell it out:

public static <E extends Enum<E>> void hello(Class<E> q)

and this will compile.

Edit: another way to think about it

You could think of fixing your method signature

public void hello(Class<? extends Enum<?>> q)

by changing it to

public void hello(Class<E extends Enum<E>> q)

just to specify that the two wildcards are the same. The concept is right, but the syntax is not legal. You must define what E is (either by making your method generic or your class).