In a Java 7 multicatch block what is the type of the caught exception?

Question

In a Java 7 multicatch block such as the following:

try {
    // code that throws exception
} catch (CharacterCodingException | UnknownServiceException ex) {
    // handle exception
}

what is the compile-time type of ex? Is it the most derived class that both exception types have in common? In this example that would be an IOException.

Answer 1

Yes, the type of ex is the most specific supertype of both CharacterCodingException and UnknownServiceException, which would be IOException.

Edit: Straight from the horse's mouth on http://cr.openjdk.java.net/~darcy/ProjectCoin/ProjectCoin-Documentation-v0.83.html#multi_catch:

Informally, the lub (least upper bound) is the most specific supertype of the types in question.

Answer 2

In JSL 7 http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.20-510

§14.20 > The declared type of an exception parameter that denotes its type as a union with alternatives D1 | D2 | ... | Dn is lub(D1, D2, ..., Dn) (§15.12.2.7).

The definition of lub() i.e. the least upper bound is quite convoluted. Fortunately types we are talking about here are usually simply non generic subclasses of Throwable, and lub() yields the most specific super class.

For a more complicated case, consider

class E1 extends Exception implements G<A>
class E2 extends Error implements G<B>

lub(E1, E2) = Throwable & G<?>