Q. The Exercise 15.2 says BinaryTree should be a subclass of Object while the handout says Collection. Besides which, it makes more sense to derive from Collection. It actually returns an error because it cannot find 'b' as a value. You are right. The vacation taken beyond what is accrued effects the salary.
Some countries have lots of laws about how payroll is calculated.
This is part of what makes payroll interesting. It is even more interesting for multinational companies, since every country will have different laws. In a later homework, you will write a method by which the accruedVacation of an employee at any point of time can be accessed. But that is not part of this homework. Actually, we have acheived the results by using the pre-existing methods.