Null 规格严格

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 == and equals must be done with care.