在这种情况下，为什么 Streams API 需要泛型类型的提示?

Why does the Streams API need a hint for generic type in this case?(在这种情况下，为什么 Streams API 需要泛型类型的提示?)

问题描述

以下编译失败:

    @NotNull String defaultFormatter(@Nullable Object value) {
        if (value instanceof Collection) {
            return ((Collection) value).stream()
                        .map(MyClass::defaultFormatter)
                        .collect(Collectors.joining(eol));
        }
        return String.valueOf(value);
    }

尤其是使用 javac 编译时，错误会是:

In particular, when compiled with javac, the error would be:

Error:(809, 94) java: incompatible types: 
      java.lang.Object cannot be converted to 
      @org.jetbrains.annotations.NotNull java.lang.String

但是下面的编译就好了:

But the following compiles just fine:

    @NotNull String defaultFormatter(@Nullable Object value) {
        if (value instanceof Collection) {
            Stream<String> stream = ((Collection) value).stream()
                         .map(MyClass::defaultFormatter);
            return stream.collect(Collectors.joining(eol));
        }
        return String.valueOf(value);
    }

唯一的区别是我引入了一个额外的变量.请注意，我没有强制转换，所以没有语义变化.

The only difference would be that I introduced an extra variable. Note that I didn't cast, so no semantic change.

谁能解释为什么需要这样做?

Can anybody explain why is this needed?

推荐答案

这个答案的顶部基本上是 Radiodef 在上面的评论中说.我不想盗用这些话，但如果没有事先说明，--- 下面的答案实际上是行不通的.

This top part of this answer is basically what Radiodef said in comments above. I'm not wanting to steal those words, but the answer below the --- doesn't really work without the prior explanation.

正如 Radiodef 所指出的，这在第一种情况下不起作用的原因是因为它使用的是原始类型 Collection.相反，使用 Collection<?>，它会起作用:

As pointed out by Radiodef, the reason why this doesn't work in the first case is because it's using a raw type, Collection. Instead, use Collection<?>, and it will work:

        return ((Collection<?>) value).stream()
                    .map(MyClass::defaultFormatter)
                    .collect(Collectors.joining(eol));

它与显式变量一起工作的原因是未经检查的转换.请注意，以下内容会产生未经检查的转换警告:

The reason why it works with the explicit variable is because of unchecked conversion. Note that the following produces an unchecked conversion warning:

        Stream<String> stream = ((Collection) value).stream()
                     .map(MyClass::defaultFormatter);

RHS上表达式的实际类型是Stream；您可以将其强制转换为 Stream<String>，如 JLS Sec 5.1.9:

The actual type of the expression on the RHS is Stream; you're allowed to coerce that to a Stream<String>, as described in JLS Sec 5.1.9:

存在从原始类或接口类型(第 4.8 节)G 到 G.

There is an unchecked conversion from the raw class or interface type (§4.8) G to any parameterized type of the form G<T1,...,Tn>.

<小时>

没有变量你不能做同样的事情的原因有点微妙.这个答案更直接地解决了这个问题:当您使用原始类型时，所有泛型都会被删除从类型，而不仅仅是那些与省略的类型直接相关的.

---

The reason why you can't do the same without the variable is a bit more subtle. This answer addresses the issue more directly: when you use a raw type, all generics are erased from the type, not just ones directly related to the omitted type.

所以，Stream 为 raw 时的 Stream.collect 类型是泛型时类型的擦除:

So, the type of Stream.collect when the Stream is raw is the erasure of the type when it is generic:

• Stream.collect(Collector collector) 返回一个R;
• R的擦除是Object

• Stream.collect(Collector<? super T,A,R> collector) returns an R;
• The erasure of R is Object

所以 collect 调用的返回类型是 Object，正如您在此处看到的那样.这不能通过未经检查的转换自动强制转换为 List，因为它不是 List.

so the return type of the collect call is Object, as you observe here. This can't be automatically coerced to a List<String> via unchecked conversion because it's not List.

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