Catching multiple exceptions in Java-8

Question

While trying out the multi-catch feature I found in my m1() method everything is working fine as expected.

However, in m2() the same code does not compile. I have just changed the syntax to reduce the number of lines of code.

public class Main {

    public int m1(boolean bool) {
        try {
            if (bool) {
                throw new Excep1();
            }
            throw new Excep2();
            //This m1() is compiling  abs fine.
        } catch (Excep1 | Excep2 e) {
            return 0;
        }
    }

    public int m2(boolean b) {
        try {
            throw b ? new Excep1() : new Excep2();
            //This one is not compiling.
        } catch (Excep1 | Excep2 e) {
            return 0;
        }
    }

    private static interface I {
    }

    private static class Excep1 extends Exception implements I {
    }

    private static class Excep2 extends Exception implements I {
    }
}

Why doesn't method m2() compile?

What compilation error are you getting? – Gavin Jan 09 '20 at 11:13 — Gavin, Jan 09 '20 at 11:13

score 81 · Accepted Answer · edited Jan 22 '20 at 13:05

The type of the expression

b ? new Excep1() : new Excep2()

is Exception, since that's the common supertype of Excep1 and Excep2.

However, you are not catching Exception, so the compiler complains about it.

If you catch Exception, it will pass compilation:

public int m2(boolean b) {
    try {
        throw b ? new Excep1() : new Excep2();
    } catch (Exception e) {
        return 0;
    }
}

I tried to find the JLS entry that explains the type of conditional ternary expression in your example.

All I could find was that this particular expression is a 15.25.3. Reference Conditional Expression.

I'm not entirely sure if it counts as a poly expression or a standalone expression. I think it's standalone (since poly expressions involve an assignment context or an invocation context, and I don't think a throw statement counts as either of those).

For a standalone expression: "If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression."

In your case, the second and third operands have three common types - Object, Throwable and Exception - the type of the expression must be one of the latter two, since, "The Expression in a throw statement must either denote a variable or value of a reference type which is assignable (§5.2) to the type Throwable."

It appears that the compiler picks the most specific common type (Exception), and therefore a catch (Exception e) solves the compilation error.

I also tried to replace your two custom exceptions with two sub-classes of IOException, in which case catch (IOException e) solves the compilation error.

@Smile the type of the ternary conditional expression must be common to both the 2nd and 3rd operands. Therefore it cannot be `Excep1` or `Excep2`. It can only be `Exception`. — Eran, Jan 09 '20 at 11:18
The final bullet point in 15.25.3 has the answer: "Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2. The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2)." lub here is Least Upper Bound, which is the nearest common supertype the two expressions' types share. — amalloy, Jan 10 '20 at 21:32

GideonleGrange · Answer 2 · 2020-07-05T08:01:01.217

23

You're confusing the compiler with this line:

throw b ? new Excep1() : new Excep2();

The compiler sees that the result of the expression (to the right of the throw) is the common super class between Except1 and Except2, which is Exception, and therefore the effective type you are throwing becomes Exception. The catch statement cannot pick up that you're trying to throw Excep1 or Except2.

edited Jul 05 '20 at 08:01

answered Jan 09 '20 at 11:19

GideonleGrange

336
1
8

score 4 · Answer 3 · answered Jan 09 '20 at 11:20

Java restrict you to catch or declare all exception types that method can throws,

It search for common parent for both (/all) Exceptions and expect you to catch or declare as throws, for example if Excep1 extends Throwable you will have to catch also Throwable

In first case Java is sure you are either throwing Excep1 or Excep2

Catching multiple exceptions in Java-8

3 Answers3