java - 泛型类实例的三种声明之间的区别

Question

Gen <Integer> iOb = new Gen <> (50, n);    //Works good

Gen <Integer> gens[] = new Gen <> [10];    //Error

Gen <?> gen[] = new Gen <?> [10];          //Alternative way for the second form

我想知道为什么第一个和第三个声明可以正常工作，但第二个声明却不能。

三者有什么区别？

Answer 1

第二行和第三行是创建通用数组的示例。第二个是不可具体化的。这意味着该类型在运行时不可用。

JLS, Section 15.10 ，涵盖数组创建:

It is a compile-time error if the ClassOrInterfaceType does not denote a reifiable type (§4.7).

Section 4.7定义一个可具体化的类型:

A type is reifiable if and only if one of the following holds:

• It refers to a non-generic class or interface type declaration.

• It is a parameterized type in which all type arguments are unbounded wildcards (§4.5.1).

• It is a raw type (§4.8).

• It is a primitive type (§4.2).

• It is an array type (§10.1) whose element type is reifiable.

• It is a nested type where, for each type T separated by a ".", T itself is reifiable.

因此，第二行是不允许的，因为它是不可具体化的类型的数组类型，因为它是通用的并且不全是无界通配符。第三行很好，只是因为它的泛型都是无界通配符。

第一行没问题，因为它不是数组。