The Trouble with Checked Exceptions

Needs (2003) in the title, but still an interesting read.

TL;DR:

> Anders Hejlsberg: I see two big issues with checked exceptions: scalability and versionability.

> Anders Hejlsberg: Let's start with versioning […]. Let's say I create a method foo that declares it throws exceptions A, B, and C. In version two of foo, […] foo might throw exception D. It is a breaking change for me to add D to the throws clause of that method, because existing caller of that method will almost certainly not handle that exception.

> Adding a new exception to a throws clause in a new version breaks client code. It's like adding a method to an interface. After you publish an interface, it is for all practical purposes immutable, because any implementation of it might have the methods that you want to add in the next version. So you've got to create a new interface instead. Similarly with exceptions, you would either have to create a whole new method called foo2 that throws more exceptions, or you would have to catch exception D in the new foo, and transform the D into an A, B, or C.

> Anders Hejlsberg: The scalability issue is somewhat related to the versionability issue. […] With a little example, you can show that you've actually checked that you caught the FileNotFoundException […]. Well, that's fine when you're just calling one API. The trouble begins when you start building big systems where you're talking to four or five different subsystems. Each subsystem throws four to ten exceptions. Now, each time you walk up the ladder of aggregation, you have this exponential hierarchy below you of exceptions you have to deal with. You end up having to declare 40 exceptions that you might throw. And once you aggregate that with another subsystem you've got 80 exceptions in your throws clause. It just balloons out of control.

> In the large, checked exceptions become such an irritation that people completely circumvent the feature. They either say, "throws Exception," everywhere; or—and I can't tell you how many times I've seen this—they say, "try, da da da da da, catch curly curly." They think, "Oh I'll come back and deal with these empty catch clauses later," and then of course they never do. In those situations, checked exceptions have actually degraded the quality of the system in the large.