Java"almost entirely nominally typed"是什么方式？

Question

我从 this article 读到 "Java is almost entirely nominally typed"。所以Java.

中有结构类型的部分

Java 类型系统的标称部分和结构部分是什么？

Answer 1

据我所知，Java 完全是名义上的类型。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 中声明类:

class A {
    public int value;
}

class B {
    public int value;
}

不会为您提供任何语言构造方式来利用以相同顺序声明的相等成员。

而在 C 中，您可以利用（在某些情况下）将 A 和 B 声明为 structs 并且它们具有相同的二进制布局，这意味着您可以将它们复制过来彼此形成 union 并利用重叠。

Answer 2

根据同一作者之前的 post - removal of function types:

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，数组和部分泛型是结构类型。

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。这些类型兼容性不仅仅基于它们的名称。

Java"almost entirely nominally typed"是什么方式？

In what way is Java "almost entirely nominally typed"?

java

typing