Those of you who have followed my blog, been subjected to it on a client site or have talked to me about programming will know that I have a few issues with checked exceptions.
Well, this morning I had an idea of a potentially powerful use of checked exceptions (if only temporarily).

The same people who will have heard about my dislike of checked exceptions will probably also know about my dislike of returning null. I was mulling over a legacy code base that I may be taking over in the future and one of the first things that I would like to do is to replace all null returns with something meaningful.

Normally, I would remove nulls by searching for the offending return statement, fixing it to do something meaningful (and if I don’t know what meaningful means in this place, throw an exception), then find usages of that method in the workspace and remove any (now defunct) null checks.

Enter the checked exception as a refactoring tool!


public Result someMethod() {
Result result = someMethodIDontControl();
if(null == result) {return null;} // Actually, this is really crap because in reality, I'd just return result, but bear with me!
return result;
}


becomes


public Result someMethod() throws IUsedToReturnNullPleaseFixMe {
Result result = someMethodIDontControl();
if(null == result) {throw new IUsedToReturnNullPleaseFixMe();} // This is temporary. The right thing _might_ be an exception, but we'll work that out later
return result;
}


Now we can lean on the compiler to find all the places that call this method. We can strip out any null checks (that are now unnecessary), look to see if there is something meaningful that we should be doing, and then add throws IUsedToReturnNullPleaseFixMe to the method we’ve just fixed. When we’ve propagated our exception to the top level, we’re done. What I would do at this point is probably delete the exception class, fix all the resulting compile errors by removing the throws clauses, until finally the last compilation error is the original throw new IUsedToReturnNullPleaseFixMe(); which I can now replace with something meaningful (e.g. throw a meaningful exception, return a meaningful NullResult, convert to a collection and return emptySet() etc)