When to use an assertion and when to use an exception

Question

Most of the time I will use an exception to check for a condition in my code, I wonder when it is an appropriate time to use an assertion?

For instance,

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

if(group !=null)
{
    //do something
}

Could you indicate how an assertion fits in here? Should I use an assertion?

It seems like I never use assertions in production code and only see assertions in unit tests. I do know that in most cases, I can just use exception to do the checking like above, but I want to know appropriate way to do it "professionally".

Answer 1

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.