Using Interfaces for Callbacks

• Limitations of Measurable interface:

  • Can add Measurable interface only to classes under your control

  • Can measure an object in only one way
    E.g., cannot analyze a set of savings accounts both by bank balance and by interest rate

• Callback mechanism: allows a class to call back a specific method when it needs more information

• In previous DataSet implementation, responsibility of measuring lies with the added objects themselves

• Alternative: Hand the object to be measured to a method:

 public interface Measurer
 {
    double measure(Object anObject);
 }

• Object is the "lowest common denominator" of all classes

• add method asks measurer (and not the added object) to do the measuring:
  
  public void add(Object x)
  {
     sum = sum + measurer.measure(x);
     if (count == 0 || measurer.measure(maximum) <
        measurer.measure(x))
     maximum = x;
     count++;
  }

• You can define measurers to take on any kind of measurement
  public class RectangleMeasurer implements Measurer
  {
     public double measure(Object anObject)
     {
        Rectangle aRectangle = (Rectangle) anObject;
        double area = aRectangle.getWidth() * aRectangle.getHeight();
        return area;
     }
  }

• Must cast from Object to Rectangle
  Rectangle aRectangle = (Rectangle) anObject;

• Pass measurer to data set constructor:

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)); . . .

• Note that the Rectangle class is decoupled from the Measurer interface

Click here for Example of Measurer Interface.

Suppose you want to use the DataSet class of Section 9.1 to find the longest String from a set of inputs. Why can't this work?

   Answer: The String class doesn't implement the Measurable interface.

Why does the measure method of the Measurer interface have one more parameter than the getMeasure method of the Measurable interface?

   Answer: A measurer measures an object, whereas getMeasure measures "itself", that is, the implicit parameter.