Uninitialized int vs Integer

Question

I was just studying up on my Java in preparation for an exam and I ran into a sort of problem with uninitialized int/Integer values.

class A     {        int x;        Integer y;        static int z;        static Integer z2;        public A(){}        } 

Lets say I initialize an object of Class A. A a = new A();

I've tried this in a compiler and got the results

a.x == 0; true a.x == null; Static Error: Bad type in comparison expression a.y == 0; java.lang.NullPointerException a.y == null; true a.z == 0; true  a.z == null; Static Error: Bad type in comparison expression a.z2 == 0; NullPointerException a.z2 == null; true 

Furthermore , I tried some more uninitialized int/Interger comparisons in an interactions pane to see if I would get different results if my x, y were not class instance variables as they are above.

int x; Integer y; x == 0; true x == null; Static Error: Bad type in comparison expression y == 0; java.lang.NullPointerException y == null; true 

However, my professor claims in a lecture that the values should be as follows:

x == 0; Uninitialized x == null; Undefined y == 0; java.lang.NullPointerException y == null; Uninitialized 

Now I don't want to doubt the one who writes the exam, but which x == 0 and y == null truth value is correct? An explanation on why would be very much appreciated, thank you.

Answer 1

• a.x == 0 - True because a.x has a default value of 0.
• a.x == null - As noted, this is a compile-time error. This follows from §15.21.3: "A compile-time error occurs if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5)." The null type isn't convertible to a number.
• a.y == 0 - This tries to unbox a.y, which is null, so it throws a NullPointerException. Unlike the above (which has a literal null), the compiler doesn't try to figure out at compile-time that a.y will be null.
• a.y == null - Again, true because a.y is initialized to null
• a.z == 0 - Same as a.x (except static)
• a.z == null - Same as a.x (except static)
• a.z2 == 0 - Same as a.y (except static)
• a.z2 == null - Same as a.y (except static)

The problem with the interactions pane is that it's up to the IDE how to implement it. If x and y are local (uninitialized) variables, all four of your last comparisons will fail to compile.