Sierra & Bates Book for SCJP 5 training states that Has-a relationship always rely on instance variables.
With the book I got a CD with simulation of the exam + got one bonus exam for free.
In the bonus exam there was a question about Has-A relationship and "Has-a relationship always rely on instance variables." was one option which was not true. Explanation was that also classes static references are considered as HAS-A relationship.