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Why ”Type Bound mismatch: The type ? extends T is not a valid substitute for the bounded parameter <E extends Enum<E>> of the type Enum<E>“？

I am trying to read the hadoop 1.0.0 source code in eclipse. I downloaded the source code first and then use ant eclipse to build the project. After that, I successfully created the project in eclipse. But there is an error Type Bound mismatch: The type ? extends T is not a valid substitute for the bounded parameter <E extends Enum<E>> of the type Enum<E> on the line 396 of Gridmix.java. The error code:

ant eclipse

Type Bound mismatch: The type ? extends T is not a valid substitute for the bounded parameter <E extends Enum<E>> of the type Enum<E>

Gridmix.java

private <T> String getEnumValues(Enum<? extends T> e)
StringBuilder sb = new StringBuilder();
String sep = "";
for (Enum<? extends T> v : e) ";

return sb.toString();



1 Answer
1

Enum itself is generic (in pure Java) with restriction on the parameter T so:

T

`Enum<T extends <Enum<T>>`


You don't have any restriction on T in your code, so compiler complains, because You may end up with T = Object, but Enum cannot accept Object as T.

T = Object

Enum

Object

T

So you have to restrict T in your code as well:

T

private <T extends Enum<T>> String getEnumValues(Enum<? extends T> e)
StringBuilder sb = new StringBuilder();
String sep = "";
for (Enum<? extends T> v : e) ";

return sb.toString();



In fact in this case the wildcard wouldn't make sense, because you cannot extend T (because you cannot extend any specific enum). But that should already compile. If not, drop the wildcard.

T

enum

I see, it's not your code. So probably some older Java didn't check this properly.

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