Strange problem with In-line If-Else block

Question

Hi!
I've discovered a very strange problem with the following code:
enum Enum {
        A, B, C
    public static void main(String[] args) {
        Enum e = Enum.C;
        Object o = e == Enum.A ? 1 : e == Enum.B ? 2 : null; // Throws NullPointerException because of the null. Why???
    }I'm using Java 6
Thanks in advance!

Answer 1

Instead of guessing what the problem is, how about we go to [the source|http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.25] and see what's actually happening.
Our line is
expr ? <int> : (expr ? <int> : <null>)Let's look at the inner conditional first. By JLS 5.1.8, we have two different types, neither of which is a reference type. The null is convertible to int, so we have two types convertible to numerics and we apply unboxing to the null, which is where your error is coming from.
Your fix works because Object is not convertible to int, so we fall through to the last case and use boxing on the ints instead of unboxing on the null.
Another way to fix this is to use Integers instead of ints. Since Integer is a reference type, we hit the third case and never try to unbox.
Object o = (e == A ? Integer.valueOf(1) : (e == B ? Integer.valueOf(2) : null))