Java“几乎完全是名义上类型化的”是以什么方式键入的？

Question

我从this article读到“Java几乎完全是名义上的类型化”。因此，Java中有结构类型化的部分

Java类型系统的标称部分和结构部分是什么

Answer 1

根据同一作者之前的一篇帖子——removal of function types：

1. There are two basic approaches to typing: nominal and structural. The identity of a nominal is based on its name; the identity of a structural type is based on what it is composed of (such as "tuple of int, int" or "function from int to float".)

Most languages pick mostly nominal or mostly structural; there are not a lot of languages that successfully mix nominal and structural typing except "around the edges." Java is almost entirely nominal (with a few exceptions: arrays are a structural type, but at the bottom there is always a nominal element type; generics have a mix of nominal and structural too, and this is in fact part of the source of many of people's complaints about generics.)

所以，数组和泛型的一部分是结构类型

我认为结构类型可能类似于<T extends A & B>是<T extends A>的超类型，或者Object[]是String[][]的超类型。这些类型的兼容性不仅仅基于它们的名称

Answer 2

据我所知，Java完全是名义上的类型化。 如果两个对象具有相同的命名类型，则它们是类型兼容的。在Java声明类中：

class A {
    public int value;
}

class B {
    public int value;
}

不提供任何语言构造来利用以相同顺序声明的相等成员

而在C语言中，你可以利用（在某些情况下）声明A&B作为struct并且它们具有相同的二进制布局，这意味着您可以将它们相互复制，形成一个union并利用这种重叠