Change program written with inheritance to composition

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:

Code:

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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

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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.

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

Code:

---

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

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

Code:

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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();

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