XTend null safe throws NullPointerException

Question

I am porting my template code to XTend. At some point I have this type of condition handling in a test case:

@Test
def xtendIfTest() {
    val obj = new FD
    if (true && obj?.property?.isNotNull) {
        return
    }
    fail("Not passed")
}

def boolean isNotNull(Object o) {
    return o != null
}
class FD {
 @Accessors
 String property
}

This works as expected as the property is null and the test will fail with "Not passed" message. But a simple change in the return type of isNotNull method to Boolean (wrapper):

def Boolean isNotNull(Object o) {
    return o != null
}

fails with a NullPointerException. Examining the generated java code for this I can see that XTend is using an intermediate Boolean object expression and that is the cause of NPE. Am I missing the point of the XTend null safe operator (?.) or I can't use a method like this after the operator?

Thanks.

Answer 1

The operator behaves properly. The exception is thrown because of the usage of a Boolean in an if-expression, which requires auto-unboxing.

If you try the following:

@Test
def xtendIfTest() {
    val Boolean obj = null
    if (obj) {
        return
    }
    fail("Not passed")
}

You will also run into a NullPointerException.

This is consistent with the Java Language Specification (https://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.8) - when auto-unboxing is required this can yield a NullPointerException:

@Test
public void test() {
    Boolean value = null;
    if (value) { // warning: Null pointer access: This expression of type Boolean is null but requires auto-unboxing
        // dead code
    }
}

Hope that helps.