Conversion from null to int possible?

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).

Conversion from null to int possible?

6 Answers