JDK8 类型推断问题

问题描述

我正在尝试运行以下代码，由于类型推断，这些代码在 JDK8 下编译得很好:

I'm trying to run the following code which is compiled fine under JDK8 thanks to type inference:

public static <A,B> B convert(A a) {
  return (B) new CB();
}
public static void main(String[] args) {
  CA a = new CA();
  CB b = convert(a); //this runs fine
  List<CB> bl = Arrays.asList(b); //this also runs fine
  List<CB> bl1 = Arrays.asList(convert(a)); //ClassCastException here
}

但是，运行它会抛出 ClassCastException: CB cannot be cast to [Ljava.lang.Object，但 CB b = convert(a) 工作正常.

However, running this throws ClassCastException: CB cannot be cast to [Ljava.lang.Object, but the CB b = convert(a) works fine.

知道为什么吗?

推荐答案

每当你创建一个带有签名的泛型方法，承诺返回调用者想要的任何东西，你就是在自找麻烦.你应该从编译器那里得到一个unchecked"警告，这基本上意味着:可能会发生意外的 ClassCastExceptions.

Whenever you create a generic method with a signature that promises to return whatever the caller wishes, you are asking for trouble. You should have got an "unchecked" warning from the compiler which basically means: unexpected ClassCastExceptions may occur.

您希望编译器进行推断

List<CB> bl1 = Arrays.asList(YourClass.<CA,CB>convert(a));

而编译器实际推断

List<CB> bl1 = Arrays.asList(YourClass.<CA,CB[]>convert(a));

据我所知，因为它更喜欢不需要 varargs 封装的方法调用(它与 pre-varargs 代码兼容).

as far as I know, because it prefers method invocations not requiring a varargs packaging (which is compatible with pre-varargs code).

这会失败，因为您的 convert 方法没有返回预期的数组类型.

This fails because your convert method does not return the expected array type.

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