Change program written with inheritance to composition

[subiedude101]

The assignment is written as:

Many programs written with inheritance could be written with composition instead, and vice versa. Rewrite class BasePlussCommissionEmployee (Fig.9.11) of the CommissionEmployee-BasePlusCommissionEmployee hierarchy to use composition rather than inheritance.

Here is what fig.9.11 looks like:

public class BasePlusCommissionEmployee extends CommissionEmployee
{
   private double baseSalary; // base salary per week

   // six-argument constructor
   public BasePlusCommissionEmployee( String first, String last, 
      String ssn, double sales, double rate, double salary )
   {
      super( first, last, ssn, sales, rate );
      setBaseSalary( salary ); // validate and store base salary
   } // end six-argument BasePlusCommissionEmployee constructor
   
   // set base salary
   public void setBaseSalary( double salary )
   {
      baseSalary = ( salary < 0.0 ) ? 0.0 : salary;
   } // end method setBaseSalary

   // return base salary
   public double getBaseSalary()
   {
      return baseSalary;
   } // end method getBaseSalary

   // calculate earnings
   @Override // indicates that this method overrides a superclass method
   public double earnings()
   {
      return getBaseSalary() + super.earnings();
   } // end method earnings

   // return String representation of BasePlusCommissionEmployee
   @Override // indicates that this method overrides a superclass method
   public String toString()
   {
      return String.format( "%s %s\n%s: %.2f", "base-salaried", 
         super.toString(), "base salary", getBaseSalary() );
   } // end method toString
} // end class BasePlusCommissionEmployee

I'm gonna be honest..I've read the chapter twice and I still have no idea where to even start. Can someone help me understand this a little better.

[subiedude101]

This is what i've come up with. Am I even in the same ball park here?

public class BasePlusCommissionEmployee
{
   private CommissionEmployee commissionEmployee; // composition
   private double baseSalary; // base salary per week

   // six-argument constructor
   public BasePlusCommissionEmployee( String first, String last, 
      String ssn, double sales, double rate, double salary )
   {
      commissionEmployee = new CommissionEmployee( first, last, ssn, sales, rate );
      setBaseSalary( salary ); // validate and store base salary
   } // end six-argument BasePlusCommissionEmployee constructor
   
   // set first name
   public void setFirstName( String first )
   {
      commissionEmployee.setFirstName( first );
   } // end method setFirstName

   // return first name
   public String getFirstName()
   {
      return commissionEmployee.getFirstName();
   } // end method getFirstName

   // set last name
   public void setLastName( String last )
   {
      commissionEmployee.setLastName( last );
   } // end method setLastName

   // return last name
   public String getLastName()
   {
      return commissionEmployee.getLastName();
   } // end method getLastName

   // set social security number
   public void setSocialSecurityNumber( String ssn )
   {
      commissionEmployee.setSocialSecurityNumber( ssn );
   } // end method setSocialSecurityNumber

   // return social security number
   public String getSocialSecurityNumber()
   {
      return commissionEmployee.getSocialSecurityNumber();
   } // end method getSocialSecurityNumber

   // set commission employee's gross sales amount
   public void setGrossSales( double sales )
   {
      commissionEmployee.setGrossSales( sales );
   } // end method setGrossSales

   // return commission employee's gross sales amount
   public double getGrossSales()
   {
      return commissionEmployee.getGrossSales();
   } // end method getGrossSales
   
   // set commission employee's rate
   public void setCommissionRate( double rate )
   {
      commissionEmployee.setCommissionRate( rate );
   } // end method setCommissionRate

   // return commission employee's rate
   public double getCommissionRate()
   {
      return commissionEmployee.getCommissionRate();
   } // end method getCommissionRate

   // set base-salaried commission employee's base salary
   public void setBaseSalary( double salary )
   {
      baseSalary = ( salary < 0.0 ) ? 0.0 : salary;
   } // end method setBaseSalary

   // return base-salaried commission employee's base salary
   public double getBaseSalary()
   {
      return baseSalary;
   } // end method getBaseSalary

   // calculate base-salaried commission employee's earnings
   public double earnings()
   {
      return getBaseSalary() + commissionEmployee.earnings();
   } // end method earnings

   // return String representation of BasePlusCommissionEmployee
   @Override
   public String toString()
   {
      return String.format( "%s %s\n%s: %.2f", "base-salaried", 
         commissionEmployee.toString(), "base salary", getBaseSalary() );
   } // end method toString
} // end class BasePlusCommissionEmployee

I get the following errors when I try to compile it though:

BasePlusCommissionEmployee.java:3: cannot find symbol
symbol  : class CommissionEmployee
location: class BasePlusCommissionEmployee
   private CommissionEmployee commissionEmployee; // composition
           ^
BasePlusCommissionEmployee.java:11: cannot find symbol
symbol  : class CommissionEmployee
location: class BasePlusCommissionEmployee
         new CommissionEmployee( first, last, ssn, sales, rate );
             ^
BasePlusCommissionEmployee.java:90: operator + cannot be applied to double,CommissionEmploy
ee.earnings
      return getBaseSalary() + commissionEmployee.earnings();
                          ^
BasePlusCommissionEmployee.java:90: incompatible types
found   : <nulltype>
required: double
      return getBaseSalary() + commissionEmployee.earnings();