Why do I have to give an identifier?

Question

In code:

try
{
    System.out.print(fromClient.readLine());
}
catch(IOException )//LINE 1
{
     System.err.println("Error while trying to read from Client");
}

In code line marked as LINE 1 compiler forces me to give an identifier even though I'm not using it. Why this unnatural constrain? And then if I type an identifier I'm getting warning that identifier isn't used. It just doesn't make sense to me, forcing a programmer to do something unnecesarry and surplus. And after me someone will revise this code and will be wondering if I didn't use this variable on purpouse or I just forgot. So in order to avoid that I have to write additional comment explaining why I do not use variable which is unnecessary in my code.
Thanks

Answer 1

The identifier name is required to make parsing simpler for the compiler. And omitting the exception in a catch clause is considered bad practice IMHO rarely a good idea - in production code you should (almost always) print / log it and/or rethrow it. So not using the identifier name should be the exception (no pun intended) rather than the rule.

But if you really have a good reason to omit not use it, you can add a comment to explain your point, [Update2] and/or to tell your IDE to suppress that kind of inspection for that specific code block (most IDEs allow this) [/Update2].

Update: OK, "bad practice" may be too strong a word :-) Let me try to explain my point better.

What I mean is that typically when you catch an exception, it is preferred to try to log / use as much information out of it as you can. Which implies that you actually refer to the exception variable.

In your example above, if you only log "Error while trying to read from Client", that is a very limited amount of information. The actual cause of the problem might be (just a few guesses) a corrupted file, a network error, a formatting error ... Logging the data within the IOException would provide much more detail about it, making the problem easier to fix.

Answer 2

The compiler is correct in calling you on this. According to the Java grammar, the "parameter" to a catch clause must have both a type and a variable identifier. If it is omitted, your program text is no longer a syntactically correct Java program.

CatchClause: catch ( FormalParameter ) Block

FormalParameter: [final] Type VariableDeclaratorId

As for why things are this way - I think it's easier to parse, and allowing an option here doesn't provide you with anything. It is very common to react to an error by making use of the exception object, so the typical case is to have a variable, not to omit it. Since there is no added performance cost, they probably just went with allowing you to not use it. In my experience no IDE warns you about an unused variable if you don't refer to the exception object within your catch block.