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.