How do I get a calculation from an Interface?

Question

Money calculateGrossPay();
is part of my interface, I have to calculate the grossPay and I can't convert between double and Money.
Can you tell me how?
I don't have the option of changing the rate to a double. I have to make it work like this.
private Money rate;
private double hoursWorked;
public Money calculateGrossPay()
return grossPay;
}

Answer 1

This is the whole code for my interface. Basically it sets up a salaried employee and an hourly employee. I have to code the methods to calculate their gross pay, net pay, taxes....
public interface Employee
* Return the first name of the employee.
* @return the first name.
String getFirstName();
* Change the first name of the employee to the given parameter.
* @param firstName the new first name.
void setFirstName(String firstName);
* Return the last name of the employee.
* @return the last name.
String getLastName();
* Change the last name of the employee to the given parameter.
* @param lastName the new last name.
void setLastName(String lastName);
* Return the ID of the Employee.
* @return the ID.
String getId();
* Change the ID of the employee to the given parameter.
* @param id the new ID.
void setId(String id);
* Return the number of tax-related dependents.
* @return the count of dependents.
int getDependents();
* Change the number of dependents for this employee.
* @param dependents the new dependent count.
void setDependents(int dependents);
* Calculate the gross (pre-tax) pay for this employee.
* @return the pre-tax pay for this employee.
Money calculateGrossPay();
* Calculate the taxes (based on pay and dependents) for this
* employee.
* @return the amount of tax that should be deducted from the gross pay.
Money calculateTaxes();
* Calculate the net pay required. Net pay is the result of subtracting
* the taxes from the gross pay.
* @return the net pay for this employee.
Money calculateNetPay();
* The number of pay periods in a year. One per week.
static final int PAY_PERIODS = 52;
* The number of "regular" hours in a pay period. A standard
* forty hour work week is the default.
static final double HOURS_PER_PERIOD = 40;
}