Question

I know that when I read the answer to this I will see that I have overlooked something
that was under my eyes. But I have spent the last 30 minutes trying to figure it out myself
with no result.

So, I was writing a program in Java 6 and discovered some (for me) strange feature.
In order to try and isolate it, I have made two small examples.
I first tried the following method:

private static int foo()
{
    return null;
}

and the compiler refused it: Type mismatch: cannot convert from null to int.

This is fine with me and it respects the Java semantics I am familiar with.
Then I tried the following:

private static Integer foo(int x)
{
    if (x < 0)
    {
        return null;
    }
    else
    {
        return new Integer(x);
    }
}

private static int bar(int x)
{
    Integer y = foo(x);

    return y == null ? null : y.intValue();
}

private static void runTest()
{
    for (int index = 2; index > -2; index--)
    {
        System.out.println("bar(" + index + ") = " + bar(index));
    }
}

This compiles with no errors! But, in my opinion, there should be a type conversion error
in the line

    return y == null ? null : y.intValue();

If I run the program I get the following output:

bar(2) = 2
bar(1) = 1
bar(0) = 0
Exception in thread "main" java.lang.NullPointerException
    at Test.bar(Test.java:23)
    at Test.runTest(Test.java:30)
    at Test.main(Test.java:36)

Can you explain this behaviour?

Update

Thank you very much for the many clarifying answers. I was a bit worried because
this example did not correspond to my intuition. One thing that disturbed me was
that a null was being converted to an int and I was wondering what the result would
be: 0 like in C++? That would hae been very strange.
Good that the conversion is not possible at runtime (null pointer exception).

Answer 1

Let’s look at the line:

return y == null ? null : y.intValue();

In a ? : statement, both sides of the : must have the same type. In this case, Java is going to make it have the type Integer. An Integer can be null, so the left side is ok. The expression y.intValue() is of type int, but Java is going to auto-box this to Integer (note, you could just as well have written y which would have saved you this autobox).

Now, the result has to be unboxed again to int, because the return type of the method is int. If you unbox an Integer that is null, you get a NullPointerException.

Note: Paragraph 15.25 of the Java Language Specification explains the exact rules for type conversions with regard to the ? : conditional operator.