Problem with interfaces

Question

Greetings everyone!!! Todays' question has to do with interfaces. Anyone who might be able to help is welcomed!!!
This is my main, which constracts new object of the type Rectangle and employee and tries to find the average and the maximum area and salary accordingly
import java.awt.Rectangle;
public class DataSetTester
     public static void main(String[] args)
          Measurer m = new RectangleMeasurer();
          DataSet data = new DataSet(m);
          data.add(new Rectangle(5,10,20,30));
          data.add(new Rectangle(10,20,30,40));
          data.add(new Rectangle(20,30,5,10));
          System.out.println("Average area = " + data.getAverage());
          Rectangle max = (Rectangle) data.getMaximum();
          System.out.println("Max area Rectangle = " + max);
                                Measurer x = new EmployeeMeasurer();
          DataSet data1 = new DataSet(x);
          data1.add(new employee("Mike", 1500,10));
          data1.add(new employee("fgh",2000,10));
          employee max1 = (employee) data.getMaximum();
          System.out.println("Max employee = " + max);
}This is the DataSet class which provides the methods to find the average and the maximum
public class DataSet
     public DataSet(Measurer aMeasurer)
          sum = 0;
          count = 0;
          maximum = null;
          measurer = aMeasurer;
     public void add(Object x)
          sum = sum + measurer.measure(x);
          if(count == 0 || measurer.measure(maximum) < measurer.measure(x))
               maximum = x;
          count++;
     public double getAverage()
          if(count == 0)
               return 0;
          else
               return sum / count;
     public Object getMaximum()
          return maximum;
     private double sum;
     private Object maximum;
     private int count;
     private Measurer measurer;
}This is my interface
public interface Measurer
     double measure(Object anObject);
}The classes that implement the interface
import java.awt.Rectangle;
public class RectangleMeasurer implements Measurer
     public double measure(Object anObject)
          Rectangle aRectangle = (Rectangle) anObject;
          double area = aRectangle.getWidth() * aRectangle.getHeight();
          return area;
public class EmployeeMeasurer implements Measurer
     public double measure(Object anObject)
          employee anemployee = (employee) anObject;
          double payment = anemployee.getsalary() * anemployee.getmonths();
          return payment;
}And the class employee
public class employee
     private String name;
     private double salary;
     private double months;
     public employee()
          name = "";
          salary = 0;
          months = 0;
     public employee(String newName, double newSalary, double newMonths)
          name = newName;
          salary = newSalary;
          months = newMonths;
     public String getname()
          return name;
     public double getsalary()
          return salary;
     public double getmonths()
          return months;
}Everything compiles but when I execute I get the desired results for the rectangle class(average and maximum)
, but it does not work for employee.
I get: Exception in thread main java.lang.ClassCastException: java.awt.Rectangle
at DataSetTester.main(DataSetTester.java:18)
Edited by: mixalissen on 26 ??? 2008 10:29 ??

Answer 1

max,
What you are constructing is called a "dirty" collection... as the negative connotation suggests, that isn't the recommended practice.
The recommended practice is to treat "disparate things" as distinct types... and if you require collections of those things then you create two individual and distinct collections.
However, in your example, Employee and Rectangle have "behavior in common" so you can (even if it doesn't make a lot of sense in "the real world") create an interface, which (among other things) allows you to create a collection of the-type-of-that-interface.
So your DataSet should contain Measurable 's (note: not Measurer's)... and Rectangle and Employee should both implement Measurable (hint: think Rectangle is Measurable; not Rectangle is a Measurer; and never RectangleMeasurer (yuck))... and then of course you'll need to move the appropriate measure method to Rectangle and Employee.
Cheers. Keith.