Java 泛型中的类型推断

这样写，编译器报错：

jshell> List unknows = new ArrayList()
|  Error:
|  unexpected type
|    required: class or interface without bounds
|    found:    ?
|  List unknows = new ArrayList();
|                        ^----------^

可以这样写：

jshell> List unknows = new ArrayList()
unknows ==> []