You are using OvertimePay, GrossPay and RegularPay without using their getters, and these properties haven't been initilizated. You should call the getters.
import java.text.DecimalFormat;
public class Employee
{ // begin class
// *********** CLASS VARIABLES **********
// *********** CLASS CONSTANTS **********
private static int MAXHOURS = 40; // maximum hours before overime
private static double OTRATE = 1.5; // overtime is one and a half
// ********** INSTANCE VARIABLES **********
private int ID; // employee's id
private int Hours; // number of hours worked
private double Wage; // pay per hour
private double RegularPay; // regular pay
private int OverHours; // number of overtime hours worked
private double OverPay; // overtime pay
private double GrossPay; // gross pay
// ********** CREATE INPUT STREAMS **********
DecimalFormat df1 = new DecimalFormat ("#####.00"); // to get two decimal places at the end of the numbers
// ********** CONSTRUCTORS ***********
public Employee(int IDnumber)
{ // begin initialized constructor
ID = IDnumber; // set ID to ID number
} // end initialized constructor
// ********** ACCESSORS **********
public int getID()
{ // begin getID
return ID;
} // end getID
public void setWage(double HourlyWage)
{ // begin setWage
Wage = HourlyWage;
} // end setWage
public double getWage()
{ // begin getWage
return Wage;
} // end getWage
public void setHours(int hoursWorked)
{ // begin setHours
Hours = hoursWorked;
} // end setHours
public double getHours()
{ // begin getHours
return Hours;
} // end getHours
// ********** MUTATORS **********
public double getOverPay()
{ // begin getOverPay
if (Hours > MAXHOURS)
{ // begin if hours worked is bigger than MAXHOURS
OverHours = Hours - MAXHOURS;
OverPay = OverHours * Wage * OTRATE;
} // end if hours worked is bigger than MAXHOURS
else
OverPay = 0;
return OverPay;
} // end getOverPay
public double getRegularPay()
{ // begin getRegularPay
return MAXHOURS * Wage;
} // end getRegularPay
public double getGrossPay()
{ // begin getGrossPay
return getRegularPay() + OverPay;
} // end getGrossPay
public String toString() // overrides the toString method inherited from object
{ // begin toString
String strout = "\nId \t\t Hours \t\t Rate \t\t Regular Pay \t Overtime Pay \t Gross Pay\n";
strout += ID + "\t " + Hours + "\t\t\t $" + (df1.format(Wage)) + "\t\t $" + (df1.format(getRegularPay())) + "\t\t\t $" + (df1.format(getOverPay())) + "\t\t\t $" + (df1.format(getGrossPay()));
// df1.format(double) allows me two decimal places
return strout;
} // end toString
} // end class