In the first case, the compiler knows that you're trying to unbox a compile-time constant of null.
In the second case, the type of the conditional expression is Integer, so you're effectively writing:
Integer tmp = new Random().nextBoolean() ? 1 : null;
return (int) tmp;
... so the unboxing isn't happening on a constant expression, and the compiler will allow it.
If you changed it to force the conditional expression to be of type int by unboxing there, it would fail:
// Compile-time failure
return new Random().nextBoolean() ? 1 : (int) null;
Boxing partially hides the distinction between primitives and corresponding wrapper objects, but it doesn't remove it.
There are two distinctions which are not changed by boxing:
- objects can be null, while primitives cannot
- objects have both state and identity, while primitives have only state (the value)
Occasionally, these differences can cause problems when using boxing.
Some points to remember :
- be careful with nulls. Auto-unboxing a null object will cause a
NullPointerException. - comparing items with
==andequalsmust be done with care.