Java中的checked exception和unchecked exception

Java中有两种异常：checked exception和unchecked exception。

checked exception

checked exception是这样定义：

A checked exception is an exception that must be either caught or declared in a method where it can be thrown.

也就是说，我们在编写代码时必须要捕获或者抛出的异常，它在编译时期就会被发现，强制要求做相应的处理，Checked exceptions子类是继承 java.lang.Exception，像FileNotFoundException，ParseException等都是checked exception，必须要使用try…catch（或者throws）来捕获异常。

unchecked exception

unchecked exception是这样定义的;

Unchecked, uncaught or runtime exceptions are exceptions that are not required to be caught or declared, even if it is allowed to do so.

unchecked exception继承自RuntimeException或Error ，unchecked exception指的是本该一切正常的程序发生了不该发生的异常，比如ArrayIndexOutOfBoundException, ClassCastException，NullPointerException,程序不该去catch这类异常，这类异常一般来说是代码有问题，需要修复。

那么我们什么时候选择checked exception，什么时候选择unchecked exception呢？官方文档给出了以下规则：

Here’s the bottom line guideline: If a client can reasonably be expected to recover from an exception, make it a checked exception. If a client cannot do anything to recover from the exception, make it an unchecked exception.