异常的Root Class是Throwable，Throwable派生了Error和Exception。
Java 8 API Doc中对checked exception和unchecked exception 的说明：
1. checked exception：（在Exception类中的说明）
The class Exception and any subclasses that are not also subclasses of RuntimeException are checked exceptions.
2. unchecked exception：
（在Error类中说明）
Error and its subclasses are regarded as unchecked exceptions for the purposes of compile-time checking of exceptions
（在RuntimeException类中的说明）
RuntimeException and its subclasses are unchecked exceptions.
3. checked exception：编译器要求对其进行显示的捕获或抛出，例如，IOException，SQLException；
unchecked exception：一般发生在运行期，编译器不要求对其进行显示的捕获或抛出，例如NullPointerException，ClassCastException；
RuntimeException and its subclasses are unchecked exceptions.