泛型:通配符类型的编译错误

Question

我已经切换到Java 7 build 21并开始出现奇怪的编译错误.例如,下面的代码片段无法编译(尽管IntelliJ不显示任何错误):

    1 Iterable<?> parts = ImmutableList.<String>of("one", "two");
    2 Function<?, String> function = new Function<Object, String>() {
    3    @Override
    4    public String apply(final Object input) {
    5        return input.toString();
    6    }
    7 };
    8 Iterable<String> result = Iterables.transform(parts, function);
    9 System.out.println(result);

但如果我?在第2行更换Object

    2 Function<Object, String> function = new Function<Object, String>() {

然后编译成功.

我得到的错误有点神秘:

error: method transform in class Iterables cannot be applied to given types;
required: Iterable<F>,Function<? super F,? extends T> 
found: Iterable<CAP#1>,Function<CAP#2,String> 
reason: no instance(s) of type variable(s) F,T exist so that argument type Function<CAP#2,String>
conforms to formal parameter type Function<? super F,? extends T> 
where F,T are type-variables:
F extends Object declared in method <F,T>transform(Iterable<F>,Function<? super F,? extends T>) 
T extends Object declared in method <F,T>transform(Iterable<F>,Function<? super F,? extends T>) 
where CAP#1,CAP#2 are fresh type-variables: 
CAP#1 extends Object from capture of ?
CAP#2 extends Object from capture of ? extends Object

改变第2行

    2 Function<? extends Object, String> function = new Function<Object, String>() {

没有效果.

我正在使用JDK 1.7.0_11-b21; 这用于编译与构建4的确定.

这是一个javacbug还是我的？

Answer 1

方法签名是:

<F,T> Iterable<T> transform(Iterable<F> fromIterable, Function<? super F,? extends T> function) 

这对于类型参数F的意义是:

Iterable<F> :
    F can be any type here, but will influence the type parameter to Function
Function<? super F, ? extends T> :
    the first type parameter (? super F) MUST be a supertype of F

键入时:

Iterable<?>
Function<?, String>

你说任何东西都可以,所以它可以是例如Iterable<Integer>.你也说从任何东西到String的函数,所以它可能是例如Function<String, String>.由于String不是Integer的超类,因此您无法满足(? super F)条件.