Why is Java auto-boxing harmful?

This is why:

        Integer version = (isSomeCondition? someObject.getInteger() : -1);

An innocently looking piece of code that totally makes you pay for the things you don't know. How?

When the return value of someObject.getInteger()is null the code will cause a NullPointerException because the JVM would first un-box the Integer to make it match the type of the -1 (int).

One thing I wonder is which Java rule makes the JVM prefer un-boxing the someObject.getInteger() to boxing the -1?

Drop me line if you know.