blocks|key|106739|text|即使该文件现在存在，但在您的系统中，以后也可能不存在。或者，您可以将此代码提供给没有src/square.jpg的人。或者可能会出现硬件故障，硬盘上的某些东西被损坏，不小心删除了src/src.src。也许用户甚至可以删除这些文件。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|106740|Java中的异常处理迫使您考虑在最坏的情况下，如果发生了一些非常糟糕的事情会发生什么(比如src/square.jpg丢失了)。你会撞车吗？继续下去就像什么都没发生一样好吗？您将在catch子句中决定如何处理这些故障模式。|106741|有些函数不要求您处理异常，因为在可以合理地期望您处理的函数中没有任何可能出错的地方。|106742|entityMap^0|16|E|0|19|E|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|R|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|K|$]]

Even if the file exists now, on your system, it might not exist later. Or you may give this code to someone who doesn't have <code>src/square.jpg</code>. Or maybe there will be a hardware malfunction where something on your hard drive gets corrupted and accidentally deletes src/square.jpg. Maybe the user might even just delete the files.

Exception handling in Java forces you to think about what would happen in the worst case if something really bad happens (like <code>src/square.jpg</code> goes missing). Do you crash? Is it okay to continue on like nothing happened? You get to decide how to handle these failure modes in the catch clause.

Some functions don't require you to handle exceptions because there isn't really anything that could go wrong in the functions that you could be reasonably expected to handle.

blocks|key|106771|text|当文件确实存在时，为什么我必须添加一个ExceptionHandler？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|106772|基本上必须添加它，因为您不能编写这样的条件代码，简而言之，编译器不可能在运行时之前知道该文件是否存在，因此编译器强制yo放置一个/catch块，因为FileNotFoundException是一个+已选中+异常。|offset|length|style|BOLD|106773|为什么我必须为某些函数而不是其他函数添加一个ExceptionHandler？|106774|**NOT*只需将try/catch块添加到抛出检查异常的任何内容，即**NOT*从RuntimeException或Error类继承的任何内容。Error和RuntimeException的子类不检查异常，您可以将try/catch放到编译器不关心的位置。由于ImageIcon的构造函数不抛出任何类型的异常，如果图像不存在，则只返回null，因此不需要执行try/catch块。|CODE|106775|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/docs/books/jls/second_edition/html/exceptions.doc.html^0|0|0|1S|2V|3|2R|3|0|0|0|13|0|0|6|10|5|16|G|1N|5|21|5|27|G|3N|9|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]|$D|Y|E|Z|F|G]]|9|@$D|10|E|11|1|12]]|A|$]]|$1|H|3|I|5|6|7|13|8|@$D|14|E|15|F|G]]|9|@]|A|$]]|$1|J|3|K|5|6|7|16|8|@$D|17|E|18|F|G]|$D|19|E|1A|F|G]|$D|1B|E|1C|F|L]|$D|1D|E|1E|F|L]|$D|1F|E|1G|F|L]|$D|1H|E|1I|F|L]|$D|1J|E|1K|F|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

Why do I have to add an ExceptionHandler when the file do exist?

Basically you have to add it regardless, because you cannot write conditional code like that, in short there is no way that for the compiler to know before runtime if the file exists or not, therefore the compiler forces yo to put a try/catch block, since FileNotFoundException is a <a href="http://java.sun.com/docs/books/jls/second_edition/html/exceptions.doc.html" rel="nofollow">checked</a> exception.

Why do I have to add an ExceptionHandler for some functions and not others?

You only have to add try/catch blocks to anything that throws a checked exception, that is anything that does **NOT* inherit from <code>RuntimeException</code> or <code>Error</code> classes. Subclasses of <code>Error</code> and <code>RuntimeException</code> are not checked exceptions and you may either put the try/catch or not the compiler does not care. Since the constructor for <code>ImageIcon</code> does not throw any kind of exceptions and will simply return null if the image does not exist there is no need to do a try/catch block.*

blocks|key|588382|text|1)当文件存在时，为什么要添加ExceptionHandler？|type|blockquote|depth|inlineStyleRanges|entityRanges|data|588383|因为Java编译器无法知道该文件在某些任意运行时是否实际存在。(假设在编译后但在运行程序之前删除了该文件？)基本上，您的代码必须始终具有处理条件所需的逻辑，这些条件是不希望发生的，但无论如何也可能发生。|unstyled|588384|2)为什么我必须为某些函数而不是其他函数添加一个ExceptionHandler？|588385|可能有各种原因，但这是你可能看到的原因。每个方法都会抛出异常，但如果将异常向上传播，则每个try/catch块或每个方法都会被捕获。在您的示例中，您可以将对ImageIcon构造函数的每个调用包装在自己的try/catch块中，或者将两者结合在一起，这取决于您想要做什么：|offset|length|style|CODE|588386|try+{
++icon1+=+new+ImageIcon("f1.jpg");
}+catch+(Exception+e)+{+/*+Handle+the+case+for+missing+"f1.jpg".+*/+}
try+{
++icon2+=+new+ImageIcon("f2.jpg");
}+catch+(Exception+e)+{+/*+Handle+the+case+for+missing+"f2.jpg".+*/+}|code-block|syntax|javascript|588387|与之相比：|588388|try+{
++icon1+=+new+ImageIcon("f1.jpg");
++icon2+=+new+ImageIcon("f2.jpg");
}+catch+(Exception+e)+{+/*+Handle+the+case+for+missing+"f1"+or+"f2".+*/+}|588389|entityMap^0|0|0|0|26|9|0|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Y|8|@]|9|@]|A|$]]|$1|E|3|F|5|6|7|Z|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|10|8|@$I|11|J|12|K|L]]|9|@]|A|$]]|$1|M|3|N|5|O|7|13|8|@]|9|@]|A|$P|Q]]|$1|R|3|S|5|D|7|14|8|@]|9|@]|A|$]]|$1|T|3|U|5|O|7|15|8|@]|9|@]|A|$P|Q]]|$1|V|3|-4|5|D|7|16|8|@]|9|@]|A|$]]]|W|$]]

<blockquote>
 1) Why do I have to add an ExceptionHandler when the file do[es] exist?
</blockquote>

Because the Java compiler cannot know whether or not that file will actually exist at some arbitrary runtime. (Suppose the file was deleted after you compiled but before you ran the program?) Basically, your code must always have the logic required to handle conditions which aren't expected to happen but might anyway.

<blockquote>
 2) Why do I have to add an ExceptionHandler for some functions and not others?
</blockquote>

There could be various reasons but here's the one you're probably seeing. Exceptions are thrown per method but caught per try/catch block or per method if they are propagated upwards. In your example, you could wrap each call to the <code>ImageIcon</code> constructor in its own try/catch block or both together, depending on what you want to do:

<pre><code>try {
 icon1 = new ImageIcon("f1.jpg");
} catch (Exception e) { /* Handle the case for missing "f1.jpg". */ }
try {
 icon2 = new ImageIcon("f2.jpg");
} catch (Exception e) { /* Handle the case for missing "f2.jpg". */ }
</code></pre>

Compared to:

<pre><code>try {
 icon1 = new ImageIcon("f1.jpg");
 icon2 = new ImageIcon("f2.jpg");
} catch (Exception e) { /* Handle the case for missing "f1" or "f2". */ }
</code></pre>

blocks|key|285880|text|因为Java区分了所谓的“检查”和“未检查”异常。对于是否应该存在未经检查的异常，以及API方法是否应该抛出异常，这是许多激烈争论的根源。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|285881|根据Java：|offset|length|style|BOLD|285882|这里的底线是:如果可以合理地期望客户端从异常中恢复，那么就将其作为一个检查过的异常。如果客户端无法从异常中恢复，则将其设置为未经检查的异常。|blockquote|285883|这就是所谓的理由。|285884|阅读更多关于“争议”的这里。|285885|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/tutorial/essential/exceptions/runtime.html^0|0|0|7|0|0|0|B|2|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|10|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|11|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|12|8|@]|9|@$D|13|E|14|1|15]]|A|$]]|$1|O|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

Because Java differentiates between what are known as "checked" and "unchecked" exceptions. It's the source of a lot of heated controversy, as to whether or not unchecked exceptions should even exist, and whether or not API methods should throw exceptions.

According to the Java Trails:

<blockquote>
 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.
</blockquote>

That's the so-called rationale.

Read more about "the controversy" <a href="http://docs.oracle.com/javase/tutorial/essential/exceptions/runtime.html" rel="nofollow">here</a>.

blocks|key|106864|text|当文件确实存在时，为什么我必须添加一个ExceptionHandler？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|106865|它目前是存在的，但不能保证它每次都会存在。可能是因为很多原因。如果没有文件，那么执行InputStream和相关的业务逻辑是没有意义的，而且您的业务肯定不会成功。|106866|为什么我必须为某些函数而不是其他函数添加一个ExceptionHandler？|106867|有些类，如Image图标，它们不是您的业务处理的停止。如果它们不存在，没关系，您仍然可以继续您的核心业务逻辑。|106868|我觉得这是为什么有些班级强制规定例外，而有些则不是。|106869|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|M|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|N|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|O|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|P|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|K|$]]

Why do I have to add an ExceptionHandler when the file do exist?

It exists for now, but there is no guarantee it will there every time. Could happen because of whole lot of reasons. If there is no file, it doesn't make sense to do InputStream and associated business logic and it is sure that your business won't be successful.

Why do I have to add an ExceptionHandler for some functions and not others?

Some Classes like Imageicon, they are not show stoppers for your business processing. If they are not there, that's Ok, you can still continue with your core business logic. 

I feel this is main reason why some class mandate exceptions and some are not.

blocks|key|588483|text|检查异常背后的想法是，它们使函数的调用方能够知道哪些异常可能会从异常中逸出。如果不愿意，则不必实际捕获FileNotFoundException，前提是将throws+FileNotFoundException添加到方法签名中，以便让调用方知道这样的异常可能从您的方法中逃脱。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|588484|这是一个很好的概念，但不幸的是，没有简洁的方法来说明您希望捕获所有不太严重的异常，并将它们包装为调用方的常见异常类型。在很多情况下，当异常发生时，你想传达的真正信息要么是“方法没有完成，但系统似乎没有着火，任何副作用都已经消除了”，要么是“方法没有完成，系统似乎没有着火，但可能还有其他副作用。”如果有一个简洁的语法来指示应该捕获异常并将异常包装到上述格式之一的代码区域，这将是有帮助的，但是遗憾的是，没有。|588485|entityMap^0|1F|L|25|S|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

The idea behind checked exceptions is that they make it possible for the caller of a function to know what exceptions might escape from it. You don't have to actually catch the <code>FileNotFoundException</code> if you don't want to, provided you add <code>throws FileNotFoundException</code> to your method signature to let your caller know that such an exception might escape from your method.

The concept is a good one, but unfortunately there's no concise way to state that you want to catch all exceptions that aren't overly severe and wrap them into a common exception type for your caller. In a lot of cases, when an exception occurs, the real message you want to convey is either "the method didn't complete, but the system doesn't seem to be on fire and any side-effects have been undone", or "the method didn't complete, and the system doesn't seem to be on fire, but there may have been other side-effects." It would be helpful if there were a concise syntax to indicate regions of code where exceptions should be caught and wrapped to one of the above formats, but alas there is not.

A quite theoretical question this time. So I'm using this function in Eclipse:

<pre><code>CsvReader csv = new CsvReader("src/maindroite.csv");
</code></pre>

Which can't run because "Unhandled exception type FileNotFoundException". Ok, I understand that I have to add something for the case where the file doesn't exist, at which point I usually add a few lines to catche the exception and throw it away. But my question is: why do I need to catch the exception even when the file do exist? And actually, why do I even have this Exception thing for some functions and not others?

For example, let's say I'm trying to run:

<pre><code>ImageIcon icon1 = new ImageIcon("src/square.jpg");
ImageIcon icon2 = new ImageIcon("src/circle.jpg");
</code></pre>

Where "square.jpg" exists but not "circle.jpg". The program will create icon1, but not icon2, because it can't. But I don't need to add an ExceptionHandler for the case where the image doesn't exist. What is the difference between both functions?

To sum it up:

<ul>
<li>Why do I have to add an ExceptionHandler when the file do exist?</li>
<li>Why do I have to add an ExceptionHandler for some functions and not others?</li>
</ul>

Thanks!

Java exceptions rationale

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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这一次是个相当理论性的问题。所以我在Eclipse中使用这个函数：CsvReader csv = new CsvReader("src/maindroite.csv");因为“未处理的异常类型FileNotFoundException”而无法运行。好的，我知道我必须为文件不存在的情况添加一些东西，在这种情况下，我通常会...

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