java - Java 编译器忽略了完全限定的泛型类型转换错误？

Question

泛型有意在编译时进行更强的类型检查。然而，在学习官方Java教程时，我遇到了这个:

However, in some cases the compiler knows that a type parameter is always valid and allows the cast. For example:

List<String> l1 = ...;
ArrayList<String> l2 = (ArrayList<String>)l1;  // OK

来自http://docs.oracle.com/javase/tutorial/java/generics/restrictions.html#cannotCast .

我已经尝试并测试了这一点(完全反射(reflect)了文档):

List<String> l1 = new LinkedList<String>();
ArrayList<String> l2 = (ArrayList<String>)l1;  // OK

虽然一切都非常明确(没有分配给 Object 变量，没有通配符等)，但编译器无法预测随后出现的运行时错误:

Exception in thread "main" java.lang.ClassCastException: java.util.LinkedList cannot be cast to java.util.ArrayList

100% 困惑。

Answer 1

重读该链接 - 它并不是说您可以将 LinkedList 转换为 ArrayList。它指出您可以将 String 的 List 向下转换为(大概是程序员已知的)ArrayList 类型字符串。他们试图说明的是通用类型参数之间的差异。他们的第一个示例展示了从 Integer 集合到 Number 集合的转换，这是不允许的。

Typically, you cannot cast to a parameterized type unless it is parameterized by unbounded wildcards. For example:

List li = new ArrayList<>(); List ln = (List) li; // compile-time error

However, in some cases the compiler knows that a type parameter is always valid and allows the cast. For example:

List l1 = ...; ArrayList l2 = (ArrayList)l1; // OK