Why does the Java compiler sometimes allow the unboxing of null?

Question

For example:

int anInt = null;

fails at compile time but

public static void main(String[] args) {
  for (int i = 0; i < 10;          


        

Answer 1

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.

Answer 2

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;

Answer 3

You can't assign null to an int

    int anInt = null;

Java allows this since you are not assigning null to an int

    System.out.println("" + getSomeVal()); //null was just converted to a srting and was printed

If you perform this, you can get the error

    int anInt = getSomeVal();