Question

I am building my first Java library. The code uses lots of IO methods, so as you can imagine there are lots of exceptions to be caught. But I’m wondering, should I throw the exceptions? I mean surely it’s the application writer’s job to catch the exceptions. Because even if I catch them, I don’t really know what the application would like to do in the event of this occurrence.

Should I throw IOExceptions etc, or should I catch them in the library code?

Answer 1

You should handle exceptions only if you can – i.e. if your library has sufficient knowledge on how the exception should be dealt with.

Here’s an example when you should handle an exception:

• Library receives request from user
• Library tries to read a configuration file
• There is a IOException because the config file is not found
• Library catches the exception and falls back to using a default configuration (perhaps issuing a warning to the user that there is no configuration file)

Here’s an example where you should not handle an exception

• Library receives a request from the user
• The processing request fails when trying to parse an integer from a String in the request (gets a NumberFormatException)
• Library cannot handle the exception – assuming a default value (e.g. 0) would be dangerous
• Exception is thrown back to the user for them to handle

If you are going to throw an exception to the user, it is often good practice to catch and re-throw the exception – this enables you to wrap it in your own custom exception class and potentially add extra messages / details etc.

In general remember that if you want to write a good software component then it should have fail-fast behaviour. If in doubt, you should fail and let the user know (via an exception) rather than attempt to continue on an unsound basis.