Chapter 2, Problem 88E

Add a further field, refNumber, to the Book class. This field can store a reference number for a library, for example. It should be of type String and initialized to the zero length string ("") in the constructor, as its initial value is not passed in a parameter to the constructor. Instead, define a mutator for it with the following header:

public void setRefNumber (String ref)

The body of this method should assign the value of the parameter to the refNumber field. Add a corresponding getRefNumber accessor to help you check that the mutator works correctly.