- 分类：throwable的子类
有 Error 和 Exception
- Error:一般是指与虚拟机相关的问题，如系统崩溃，虚拟机错误，内存空间不足，方法调用栈溢等。对于这类错 误的导致的应用程序中断，仅靠程序本身无法恢复和和预防，遇到这样的错误，建议让程序终止。
/**
* An {@code Error} is a subclass of {@code Throwable}
* that indicates serious problems that a reasonable application
* should not try to catch. Most such errors are abnormal conditions.
* The {@code ThreadDeath} error, though a "normal" condition,
* is also a subclass of {@code Error} because most applications
* should not try to catch it.
* <p>
* A method is not required to declare in its {@code throws}
* clause any subclasses of {@code Error} that might be thrown
* during the execution of the method but not caught, since these
* errors are abnormal conditions that should never occur.
*
* That is, {@code Error} and its subclasses are regarded as unchecked
* exceptions for the purposes of compile-time checking of exceptions.
- Exception:表示程序可以处理的异常，可以捕获且可能恢复。遇到这类异常，应该尽可能处理异常，使程序恢复运行，而不应该随意终止异常。
**
* The class {@code Exception} and its subclasses are a form of
* {@code Throwable} that indicates conditions that a reasonable
* application might want to catch.
*
* <p>The class {@code Exception} and any subclasses that are not also
* subclasses of {@link RuntimeException} are <em>checked
* exceptions</em>. Checked exceptions need to be declared in a
* method or constructor's {@code throws} clause if they can be thrown
* by the execution of the method or constructor and propagate outside
* the method or constructor boundary.
- 其中Exception可分为2种：
RuntimeException：运行期异常，系统异常
可以处理也可以不处理，编译器不强制用 try..catch 处理或用 throws 声明，所以系统异常也称为 unchecked 异常。
eg：NullPointerException,IndexOutOfBoundsException,ClassCastException,ClassNotFoundException ， IllegalArgumentException，UnsupportedOperationException。CheckedException: 也成编译器异常
编译器强制普通异常必须 try..catch 处理或用 throws 声明继 续抛给上层调用方法处理
eg：CloneNotSupportedException
