课件Java程序设计方案.ppt

Java程序设计
Java Programming
Spring, 2013
课件Java程序设计方案
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Contents
Why Exceptions?
What are Exceptions?
Handling Exceptions
Creating Exception Types
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Why Exceptions(异常)?
During execution(执行), programs can run into many kinds of errors;
What do we do when an error occurs?
Java uses exceptions to provide the error-handling capabilities for its programs.
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Exceptions (异常)
Error Class
Critical(严重的) error which is not acceptable in normal application program.
Exception Class
Possible exception in normal application program execution;
Possible to handle by programmer.
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Exceptions (异常)
An exception is an event(事件) that occurs during the execution of a program that disrupts(使中断) the normal flow(流程) of instructions(指令).
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Exceptions (异常)
Treat exception as an object .
All exceptions are instances of a class extended from Throwable class or its subclass.
Generally, a programmer makes new exception class to extend the Exception class which is a subclass of Throwable class.
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Exception Class 继承关系
Throwable
Error
Exception
RuntimeException
IOException
Object
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异常类的层次结构：
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Classifying(分类) Java Exceptions
Unchecked Exceptions(非检查性异常)
It is not required that these types of exceptions be caught or declared on a method.
Runtime exceptions can be generated by methods or by the JVM itself.
Errors are generated from deep within the JVM, and often indicate a truly fatal state.
Checked Exceptions(检查性异常)
Must either be caught by a method or declared in its signature by placing exceptions in the method signature.
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Exception的分类
非检查性异常(unchecked exception)：
以RuntimeException为代表的一些类，编译时发现不了，只在能运行时才能发现。
检查性异常(checked exception)：
一般程序中可预知的问题，其产生的异常可能会带来意想不到的结果，因此Java编译器要求Java程序必须捕获或声明所有的非运行时异常。
以IOException为代表的一些类。如果代码中存在检查性异常，必须进行异常处理，否则编译不能通过。如：用户连接数据库SQLException、。
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