java - 关于 Java 中通用数组的创建

Question

刚找到一行代码，不太明白。

List<String>[] stringLists = new List[1];

虽然有警告，但这一行可以在我的 AndroidStudio IDE 上通过编译。这似乎违反了面向对象语言的基本规则:“父类(super class)对象可以用子类实例实例化，反之则不行。”

String[] ss = new Object[1];  // won't compile

我们知道 List 是任何泛型 List 类型的父类(super class)型，例如 List<String>在这种情况下。由于数组是协变的，所以我认为 List[] 类型是 List<String>[] 的 super 类型类型。为什么 List<String>[]可以用 List[] 实例化？

Answer 1

我认为 JLS, §4.8, "Raw Types"回答您关于为什么分配有效的实际问题:

The use of raw types is allowed only as a concession to compatibility of legacy code. The use of raw types in code written after the introduction of generics into the Java programming language is strongly discouraged. It is possible that future versions of the Java programming language will disallow the use of raw types.

这是为了与前泛型代码兼容。一般来说，如果你想要类型安全，你根本不应该使用参数化类型的数组，如果作者写道:

List<List<String>> lists = new ArrayList<>();

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您关于违反类型安全规则的假设是错误的。你是说 List不是 List<String> 的子类型.但是，在 Java 类型系统中，问题的答案是:“List 是 List<String> 的子类型吗？”既不是"is"，也不是“否”。是“这个问题无法回答”。

_{(将“类型”和“类”混为一谈可能也不正确。只有一个 List 类，但 List<String> 和 List<Object> 是不同的类型。奖励:List<String> 是 < strong>不是 List<Object> 的子类型。)}

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为了添加更多关于正在发生的事情的细节，JLS 在 §5.2, "Assignment Contexts" 中解释了变量赋值中允许的转换。 .值得注意的是，该列表以:

If, after the conversions listed above have been applied, the resulting type is a raw type (§4.8), an unchecked conversion (§5.1.9) may then be applied.

后者链接到§5.1.9, "Unchecked Conversions" ，在一些形式主义解释了什么是未经检查的转换之后，重申了基本原理(强调我的):

Unchecked conversion is used to enable a smooth interoperation of legacy code, written before the introduction of generic types, with libraries that have undergone a conversion to use genericity (a process we call generification). In such circumstances (most notably, clients of the Collections Framework in java.util), legacy code uses raw types (e.g. Collection instead of Collection<String>). Expressions of raw types are passed as arguments to library methods that use parameterized versions of those same types as the types of their corresponding formal parameters.

Such calls cannot be shown to be statically safe under the type system using generics. Rejecting such calls would invalidate large bodies of existing code, and prevent them from using newer versions of the libraries. This in turn, would discourage library vendors from taking advantage of genericity. To prevent such an unwelcome turn of events, a raw type may be converted to an arbitrary invocation of the generic type declaration to which the raw type refers. While the conversion is unsound, it is tolerated as a concession to practicality. An unchecked warning is issued in such cases.

官方的说法确实是故意将未经检查的从原始类型到参数化类型的转换添加到语言中，尽管它可能不安全，并且仅出于兼容性原因标记有编译警告。 (Java 非常努力地确保在版本 X 中编译的代码永远不会在版本 X+1 中编译失败或停止使用。)