Understanding checked vs unchecked exceptions in Java

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Many people say that checked exceptions (i.e. these that you should explicitly catch or rethrow) should not be used at all. They were eliminated in C# for example, and most languages don’t have them. So you can always throw a subclass of RuntimeException (unchecked exception)

However, I think checked exceptions are useful – they are used when you want to force the user of your API to think how to handle the exceptional situation (if it is recoverable). It’s just that checked exceptions are overused in the Java platform, which makes people hate them.

Here’s my extended view on the topic.

As for the particular questions:

  1. Is the NumberFormatException consider a checked exception?
    No. NumberFormatException is unchecked (= is subclass of RuntimeException). Why? I don’t know. (but there should have been a method isValidInteger(..))

  2. Is RuntimeException an unchecked exception?
    Yes, exactly.

  3. What should I do here?
    It depends on where this code is and what you want to happen. If it is in the UI layer – catch it and show a warning; if it’s in the service layer – don’t catch it at all – let it bubble. Just don’t swallow the exception. If an exception occurs in most of the cases you should choose one of these:

    • log it and return
    • rethrow it (declare it to be thrown by the method)
    • construct a new exception by passing the current one in constructor

  4. Now, couldn’t the above code also be a checked exception? I can try to recover the situation like this? Can I?
    It could’ve been. But nothing stops you from catching the unchecked exception as well

  5. Why do people add class Exception in the throws clause?
    Most often because people are lazy to consider what to catch and what to rethrow. Throwing Exception is a bad practice and should be avoided.

Alas, there is no single rule to let you determine when to catch, when to rethrow, when to use checked and when to use unchecked exceptions. I agree this causes much confusion and a lot of bad code. The general principle is stated by Bloch (you quoted a part of it). And the general principle is to rethrow an exception to the layer where you can handle it.

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