Wildcards通配符

In generic code, the question mark (?), called the wildcard, represents an unknown type. 在泛型代码中，被称为通配符的问号（?）表示未知类型。The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific). 通配符可用于多种情况：作为参数、字段或局部变量的类型；有时作为返回类型（尽管更具体一些是更好的编程实践）。The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype.通配符永远不会用作泛型方法调用、泛型类实例创建或超类型的类型参数。

The following sections discuss wildcards in more detail, including upper bounded wildcards, lower bounded wildcards, and wildcard capture.以下各节将更详细地讨论通配符，包括上界通配符、下界通配符和通配符捕获。