Java NPE in ternary operator with autoboxing?

Question

I ran across a very weird NPE this morning, and reduced it to a simple example. Is this a JVM bug or correct behavior?

public class Test1 {
    class Item {
         


        

Answer 1

It happens because you are using conditional operator ?. Line

return item == null ? 1 : item.getId();

is equivalent to

int result = item == null ? 1 : item.getId();
return result;

The result is int because of the first operand in your expression. This is the reason that your code works when you explicitly wrap 1 with Integer. In this case the compiler creates something like

Integer result = item == null ? new Integer(1) : item.getId();
return result;

So, NPE happens when attempting to "cast" item.getId() (that is null) to int.