Auto-unboxing may lead to NullPointerException

This code compiles:

Integer arg = null;
int x = arg;

But it will crash at runtime with a java.lang.NullPointerException on the second line.

The problem is that a primitive int cannot have a null value.

This is a minimalistic example, but in practice it often manifests in more sophisticated forms. The NullPointerException is not very intuitive and is often little help in locating such bugs.

Rely on autoboxing and auto-unboxing with care, make sure that unboxed values will not have null values at runtime.