泛型：上限有界通配符转换/投射错误

Question

Here is the test:

@Test
public void genericsTest() {
    String s = "x";
    List<? extends String> strings = singletonList(s);
    Optional<Function<String, List<? extends String>>> optionalSelector = Optional.empty();
    optionalSelector.map(selector -> selector.apply(s)).orElse(strings);
}

And here is the error:

java: incompatible types: java.util.List<capture#1 of ? extends java.lang.String> cannot be converted to java.util.List<capture#2 of ? extends java.lang.String>

Of course, it can be fixed using the raw type trick.

    optionalSelector.map(selector -> selector.apply(s)).orElse((List)strings);

But I'd like to understand the cause of this error.

Update :

It's not a duplicate of Lists with wildcards cause Generic voodoo error which is about adding into an upper-bounded wildcard list; this topic is also well described in Wildcard Guidelines .

In my case I'm not trying to add something into the list as you can see above.

Also, the error messages are different.

Answer 1

There appears to be something wrong with the multiple levels of type inference here. With this line:

optionalSelector.map(selector -> selector.apply(s)).orElse(strings);

With the call to map , you're attempting to convert the original Optional<Function<String, List<? extends String>>> Optional<Function<String, List<? extends String>>> to a Optional<List<? extends String>> Optional<List<? extends String>> . Then, with orElse , you're attempting to convert that to a List<? extends String> List<? extends String> . It seems reasonable.

I can get it to compile without resorting to casting to raw types, by giving the intermediate step an explicit type.

Optional<List<? extends String>> intermedSelector = optionalSelector.map(selector -> selector.apply(s));
intermedSelector.orElse(strings);

This shows that if the compiler can infer the result of the call to map to be a Optional<List<? extends String>> Optional<List<? extends String>> , then the call to orElse will compile.

However, I can change the explicit type to something where the declaration of intermedSelector compiles, but the call to orElse does not compile. I've changed the explicit type of interMedSelector from Optional<List<? extends String>> Optional<List<? extends String>> to Optional<? extends List<? extends String>> Optional<? extends List<? extends String>> Optional<? extends List<? extends String>> , with an added ? extends ? extends .

Optional<? extends List<? extends String>> intermedSelector = optionalSelector.map(selector -> selector.apply(s));
intermedSelector.orElse(strings);

The error here is similar to the one you're getting:

J.java:26: error: incompatible types: List<CAP#1> cannot be converted to CAP#2
    List<? extends String> result = intermedSelector.orElse(strings);
                                                            ^
  where CAP#1,CAP#2 are fresh type-variables:
    CAP#1 extends String from capture of ? extends String
    CAP#2 extends List<? extends String> from capture of ? extends List<? extends String>

With your code, the compiler must be inferring something unexpected, yet related to, Optional<List<? extends String>> Optional<List<? extends String>> . I can't tell if the compiler's captured type is Optional<? extends List<? extends String>> Optional<? extends List<? extends String>> Optional<? extends List<? extends String>> or something else, but that appears to be why you're getting the error.

My solution

Optional<List<? extends String>> intermedSelector = optionalSelector.map(selector -> selector.apply(s));
intermedSelector.orElse(strings);

can be expressed in one statement, if I supply the proper type parameter to the call to map , so the compiler doesn't infer something unexpected.

optionalSelector.<List<? extends String>>map(selector -> selector.apply(s)).orElse(strings);