Enthuware

>>>in is a final variable. So, it cannot be reassigned.

It is not entirely true. There is special method System.setIn(InputStream in) for this purpose.

You are right. The problem statement has been updated to:
"The System.in variable cannot be reassigned to any other stream directly."
and the explanation has also been enhanced to include the setIn method.
thank you for your feedback!

How can a final variable be changed by a method? Does this have to do with the fact that the method setIn0() is labeled "native"?

That's right. A native method executes low level code directly using OS service calls, bypassing the JVM. This allows it do stuff such as manipulating memory directly.