英文:
Why do I need multi-catch blocks when Java exits the try block when the first error is encountered?
问题
当运行时错误发生时，它会退出 try 块。它只会抛出一个异常并进入 catch 块。那么为什么我需要多个 catch 块？多重 catch 块在哪里使用？
另外，我还可以使用 Throwable 来捕获任何异常。这样可以吗？
英文:
When the run time error occurs it exits the try block. It will throw only one exception and enter the catch block. Then why do I need multiple catch blocks? Where are multi-catch blocks used?
Also, I can also use Throwable to catch any exception. Is that right?
答案1
得分: 1
'compiler' 并未执行这些操作；编译器不存在。异常是运行时现象。
你的问题似乎在询问运行时异常分派程序如何选择异常处理程序：它基于类型匹配。
当你说 '进入**the** catch 块' 时，那是不正确的。它选择第一个捕获块，其参数类型与实际抛出的异常兼容赋值。
如果你总是写 `catch (Exception ex)`，那么这将捕获任何异常，因为 `Exception` 是所有异常类型的超类，如果这是第一个这样的 catch 块，将不会考虑其他块。但这并不总是合适的异常处理风格；建议更加具体。
或许这是理解多个 catch 块的好方法：你在表达“如果是 X 类型的异常就执行这个；否则，如果是 Y 类型的异常就执行另一个操作；否则，如果是 Z 类型的异常就执行第三件事”。
顺便提一下，除非你完全了解通过这样做会干扰哪些机制，你很少会想要写 `catch (Throwable th)`。
英文:
The 'compiler' isn't doing any of this; the compiler is not around. Exceptions are a runtime phenomenon.
Your question appears to be asking how the runtime exception dispatcher picks an exception handler: it is based on type matching.
When you say 'enter the catch block', that is not correct. It selects the first catch-block whose argument type is assignment-compatible with the exception actually thrown.
If you always write catch (Exception ex) then this will catch any exception, since Exception is a superclass of all exception types, and if this is the first such catch block no other will be considered. But this is not always an appropriate exception-handling style; you may be advised to be more specific.
Perhaps this is a good way to understand multiple catch-blocks: you're saying "if it's an X-type exception then do this; otherwise if it's a Y-type exception then do this other thing; otherwise if it's a Z-type exception then a third thing".
FWIW, you rarely want to write catch (Throwable th) unless you're fully aware of the mechanisms you're interfering with by so doing.
答案2
得分: 0
你可以使用多重捕获块来捕获不同类型的异常。假设你有一个实现了Throwable的自定义异常MyCoolExn。
try {
// 执行一段代码
} catch (MyCoolExn e) { // 捕获你的自定义异常
System.out.println("发生了自定义异常");
} catch (Throwable e) { // 捕获其他任何异常
System.out.println("发生了未知异常");
}
请注意，块的顺序很重要。如果交换它们，MyCoolExn将永远不会被捕获，因为Throwable块会捕获所有异常。
英文:
Well, you can use a multi catch block to in order to catch different types of exceptions. Let's say you have a custom exception MyCoolExn that implements Throwable.
try {
// run piece of code
} catch (MyCoolExn e) { // catching your custom exception
System.out.println("custom exception occurred");
} catch (Throwable e) { // catch any other exception
System.out.println("an unknown exception occurred ");
}
Note that the order of the blocks is important. If you swap them, MyCoolExn will never be caught, as the Trowable block will catch everything
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