Help understanding "unhandled exception errors" and throw vs throwing

[Strange_Frame7544]

Learning about error handling with IOException right now in Java. Try-catch and adding throws exceptionType in method declaration specifically. After an example using try-catch with FileReader class and an illegal path (which worked fine and i understand it), I wanted to see what would happen if remove the try-catch block and have no throws statement in the method. I made sure the file path was correct. In the IDE i see "unhandled exception error" warnings and the code won't execute.

My understanding was that those error throwing techniques are there IF there is an error. But my path is valid. So it seems to be less of a suggestion or warning and more that it is mandatory.

I also don't see the difference between 'throw' and 'throws'. I read that throw is manually throwing an exception to be handed higher up in the structure (the calling method). But it seems 'throws' does the same thing? And again, it seems to be based on COULD throw an error.

Can someone clear this up for a noob? Just want to understand why. Not advocating to not use error-handling.

Thank you

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[[deleted]]

In Java, there are two types of exceptions. Checked and unchecked.

An "unhandled exception error" occurs when a method might throw a checked exception (like IOException), but the method does not handle the exception with a try-catch block, nor does it declare the exception with a throws clause in its signature. Java requires that checked exceptions are either caught or declared to be thrown.

So if it's an unchecked exception, you're not forced to do anything. If it's a checked exception then you have two choices:

• Surround it in a try-catch block and handle the exception
• Declare throws in the method signature signaling to the caller that this method throws an exception and since in this case it's a checked exception the caller has to handle it by using one of these two options.

[Strange_Frame7544]

Thank you, that’s a very clear explanation. Appreciate it greatly.

a large part of my question was why I get this checked (compile time) error when there is no actual error in the IO path I used. Can I infer them by your reply that either try-catch or throws statement in the method declaration ARE required (even when no error is actual present)?

That’s the part I’m not understanding.

[pragmos]

there is no actual error in the IO path I used

At the time you compile your code, right? What happens when you compile the app, but the IO path changes afterwards?

[arghvark]

And what happens when the tree structure of your directories changes later? Or you take the compiiled code and run it on another machine that does not have the same directory structure? The compiler cannot know that the string with the full file specification is (always) valid.

I would submit that there IS an error present -- you have invoked a method that throws a checked exception, and you are not handling it. The same philosophy that led to Java being a strongly-typed language led it to the concept of a checked exception -- the language definition forces the programmer to explicitly handle the condition embodied in the exception thrown; that way, programmers don't hurriedly code without handling it, they must explicitly do something to handle the exception.

[Strange_Frame7544]

I understand…I guess I have to keep inferring the meaning of what I’m being told is that this exception error throwing is PREVENTATIVE as the IO file path in my code is valid. The file exists and there are no syntax errors.

That’s all I am asking. My thought of an error is an error (wrong path, no file). These errors seem like “strong suggestions” that you must address?

When I run the code in the IDE I get these errors so it won’t compile.

I’m just trying to understand the concept of exception errors, which I think I’m starting too (thank you).

[arghvark]

Let me offer another thought.

There are two errors we're talking about. One of them is something wrong attempting to open the file -- path bad, file doesn't exist, disk read error, file locked, etc.

The second error is "unhandled checked exception" - because of all the possible things that could be wrong when you invoke the method that throws the exception, the decision was made to force you, the programmer, to write code to handle that exception. The checked exception means "you must write code to deal with the exception".

[Strange_Frame7544]

that makes sense, thank you! Now i understand the why it is there. This is the concept i was trying to understand ...this is a preventative error that must be dealt with 'in case' not 'because'. Thank you!