When to use an assertion and when to use an exception

Question

Out of my mind (list may be incomplete, and is too long to fit in a comment), I would say:

• use exceptions when checking parameters passed to public or protected methods and constructors
• use exceptions when interacting with the user or when you expect the client code to recover from an exceptional situation
• use exceptions to address problems that might occur
• use assertions when checking pre-conditions, post-conditions and invariants of private/internal code
• use assertions to provide feedback to yourself or your developer team
• use assertions when checking for things that are very unlikely to happen otherwise it means that there is a serious flaw in your application
• use assertions to state things that you (supposedly) know to be true

In other words, exceptions address the robustness of your application while assertions address its correctness.

Assertions are designed to be cheap to write, you can use them almost everywhere and I'm using this rule of thumb: the more an assertion statement looks stupid, the more valuable it is and the more information it embeds. When debugging a program that does not behave the right way, you will surely check the more obvious failure possibilities based on your experience. Then you will check for problems that just cannot happen: this is exactly when assertions help a lot and save time.

Assertions should be used to check something that should never happen, while an exception should be used to check something that might happen.

For example, a function might divide by 0, so an exception should be used, but an assertion could be used to check that the harddrive suddenly disappears.

An assertion would stop the program from running, but an exception would let the program continue running.

Note that if(group != null) is not an assertion, that is just a conditional.

Remember assertions can be disabled at runtime using parameters, and are disabled by default, so don't count on them except for debugging purposes.

Also you should read the Oracle article about assert to see more cases where to use - or not to use - assert.

As a general rule:

• Use assertions for internal consistency checks where it doesn't matter at all if someone turns them off. (Note that the java command turns off all assertions by default.)
• Use regular tests for any kind of checks what shouldn't be turned off. This includes defensive checks that guard against potential damage cause by bugs, and any validation data / requests / whatever provided by users or external services.

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The following code from your question is bad style and potentially buggy

try {
    group = service().getGroup("abc");
} catch (Exception e) {
    //i dont log error because i know whenever error occur mean group not found
}

The problem is that you DON'T know that an exception means that the group was not found. It is also possible that the service() call threw an exception, or that it returned null which then caused a NullPointerException.

When you catch an "expected" exception, you should catch only the exception that you are expecting. By catching java.lang.Exception (and especially by not logging it), you are making it harder to diagnose / debug the problem, and potentially allowing the app to do more damage.

Well, back at Microsoft, the recommendation was to throw Exceptions in all APIs you make available publicly and use Asserts in all sorts of assumptions you make about code that's internal. It's a bit of a loose definition but I guess it's up to each developer to draw the line.

Regarding the use of Exceptions, as the name says, their usage should be exceptional so for the code you present above, the getGroup call should return null if no service exists. Exception should only occur if a network link goes down or something like that.

I guess the conclusion is that it's a bit left down to the development team for each application to define the boundaries of assert vs exceptions.