IO is very important for performance of your Java application. Ideally you should avoid IO in critical path of your application. Here are couple of Java IO best practices you can follow:
a) Use buffered IO classes instead of reading individual bytes and char.
b) Use classes from NIO and NIO2
c) Always close streams in finally block or use try-with-resource statements.
d) use memory mapped file for faster IO.
If a Java candidate doesn't know about IO and NIO, especially if he has at least 2 to 4 years of experience, he needs some reading.