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

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;