Thread: Interface Help

Hey guys. I'm starting to study Interfaces and I am sort of getting the hang of it. However, I am studying a code I got from my book and there is a specific part that I don't understand well. I put a comment besides it to see if someone could explain it to me. It's the part in the DataSet class where the Object x is added to the sum, but it goes though an invocation of the method in the Interface. Could anyone tell me why I have to put measurer.measure(x) ??? Here is the full code:

/*
 * Describes any class whose objects can measure other objects.
 */
 
public interface Measurer
{
	/**
	 * Computes the measure of an object.
	 * @param anObject the object to be measured.
	 * @return the measure.
	 */
 
	double measure (Object anObject);
 
}

import java.awt.Rectangle;
 
/**
 * Objects of this class measure rectangles by area. 
 * @author Jean
 *
 */
 
public class RectangleMeasurer implements Measurer 
{
	public double measure(Object anObject)
	{
		Rectangle aRectangle = (Rectangle) anObject;
		double area = aRectangle.getWidth() * aRectangle.getHeight();
		return area;
	}
 
}

/**
 * Computes the average of a ser of data values.
 * @author Jean
 *
 */
 
public class DataSet 
{
	private double sum;
	private Object maximum;
	private int count;
	private Measurer measurer;
 
	/**
	 * Constructs an empty data set with a given measurer.
	 * @param aMeasurer the measurer that is used to measure data values.
	 */
 
	public DataSet(Measurer aMeasurer)
	{
		sum = 0;
		count = 0;
		maximum = null;
		measurer = aMeasurer;
	}
 
	/**
	 * Adds a data value to the data set.
	 * @param x a data value.
	 */
 
	public void add(Object x)
	{
		sum = sum + measurer.measure(x); // DONT UNDERSTAND THE measure(x) PART
		if(count == 0 || measurer.measure(maximum) < measurer.measure(x))
		{
			maximum = x;
		}
		count++;
	}
 
	/**
	 * Gets the average of the added data.
	 * @return the average or 0 if no data has been added.
	 */
	public double getAverage()
	{
		if(count == 0)
		{
			return 0;
		}
		else 
		{
			return sum/count;
		}
	}
 
	/**
	 * Gets the larger of the added data.
	 * @return the maximum or 0 if no data has been added.
	 */
	public Object getMaximum()
	{
		return maximum;
	}
}

import java.awt.Rectangle;
 
/**
 * This program demonstrates the use of a Measurer.
 * @author Jean
 *
 */
 
public class DataTester 
{
	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(20,30,5,15));
 
		System.out.println("Average area:" + data.getAverage());
		System.out.println("Expected: 625");
 
		Rectangle max = (Rectangle) data.getMaximum();
		System.out.println("Maximum area rectangle: " + max);
		System.out.println("Expeted:" + "java.awt.Rectangle[x = 10, y = 20, width = 30, height = 40]");
		}
}

This is the specific method:

public void add(Object x)
	{
		sum = sum + measurer.measure(x); // DONT UNDERSTAND THE measure(x) PART
		if(count == 0 || measurer.measure(maximum) < measurer.measure(x))
		{
			maximum = x;
		}
		count++;
	}