Reducing "exception is never thrown in body of corresponding try statement" Error to a Warning

[Alex Buckley]

This kind of substantial change to the Java language demands inspection 
from all angles. Here's one. Suppose a beginning programmer writes this:

void m() {
   try {
     throw new CheckedException1();
   } catch (CheckedException2 e) {
     log(e);
   }
}

Both CheckedException1.class and CheckedException2.class are legal 
exception types, but the `2` in the catch clause is a typo.

Today the program doesn't compile, and the error is clear: 
"CheckedException2 is never thrown in body of corresponding try 
statement". Independently, there is another error: that 
"CheckedException1 must be caught or declared to be thrown". One way or 
another, the typo will be corrected.

In your proposal, the first error is replaced with a warning -- in some 
sense taking the position that CheckedException1 is unchecked in the try 
statement because you don't NEED to catch it there -- but the second 
error will still occur, because CheckedException1 is checked in the 
method body as a whole. The programmer dutifully adds a 'throws' clause 
to the method declaration; the error goes away; the programmer is happy; 
the world is worse off.

Alex

On 11/5/2019 3:29 PM, Mike Duigou wrote:
> Hello again;
> 
> Hearing no specific objections to my prior suggestion 
> (https://mail.openjdk.java.net/pipermail/jdk-dev/2019-October/003480.html) 
> to tweak compilation of checked exceptions by reducing the "exception is 
> never thrown in body of corresponding try statement" compilation error 
> to a non-fatal warning I am going to proceed with creating a JIRA issue 
> and proceeding with proposing a changeset to the compiler-dev list.
> 
> I do agree with the sentiment that this change is of somewhat limited 
> value. It is, I believe, useful and incrementally an improvement upon 
> existing behaviour and worth pursuing in absence of grander proposals.
> 
> Cheers,
> 
> 
> Mike