Lecture 15 Objectives

Lecture 15 Objectives [Nov 17]prev next

MUSICAL COMPOSITION: Goldberg Variations, var. 8-14 (from a total of 30), J. S. Bach, played by Glenn Gould.

9:07 var. 8-14

At the end of today's class you should

HAVE READ:

[476 -495]

KNOW:

understand the concept of polymorphism (you saw this, in effect, on lab 3).
understand the concept of late binding [p. 487] that enables polymorphism in Java
understand downcasting and how to protect against faulty downcasts.

BE ABLE TO:

Write derived classes whose methods override the default implementation found in a base class
Know which method is invoked, given a set of variables and the objects to which they refer.
Apply downcasting, in a safe manner, to ensure that your equals method is accurate.

DAILY QUESTION:

Given the code examples as found in nov17.daily package in SourceForge Examples (and reproduced below) write a Rectangle derived class of Figure. Modify FigureTest to show how it is constructed and used.

package nov17;

import nov17.daily.Circle;
import nov17.daily.Figure;
import nov17.daily.Triangle;

public class FigureTest {

/** Small program to show nuances for Figures. */
public static void main (String[] args) {
    Figure circle = new Circle(2);
    Figure triangle = new Triangle (3, 4, 5);

    if (triangle.sameArea(circle)) {
       System.out.println ("Figures have same area");
    }
}
}

And Figure

package nov17.daily;

/**
* Figure is a special class because its constructor has
* been marked as being protected.
*
* Thus you can't arbitrarily go and construct Figure instances.
*
* @author heineman
*
*/
public class Figure {

/** Make available to subclasses ONLY this constructor. */
protected Figure () {
}

protected double area; /** Area of a figure. */

/**
   * Return the area of the figure.
   *
   * @return area of the figure.
   */
public double getArea () {
    return area;
}

/**
   * Set the area for the figure.
   *
   * Note that only Friends can access this method.
   *
   * @param newArea newly calculated area.
   */
   protected void setArea (double newArea) {
     area = newArea;
   }

/**
   * Compare two figures by area
   *
   * @param f Figure to be compared against
   * @return true if the figures have the same area.
   */
public boolean sameArea (Figure f) {
    return getArea() == f.getArea();
}
}

and Circle

package nov17.daily;

/**
* Represents a Circle which is a figure with a known area.
*
* @author heineman
*/
public class Circle extends Figure {

/** Circle stores this information. */
int radius;

/**
   * Construct circle using radius
   *
   * @param r radius of circle
   */
public Circle (int radius) {
    this.radius = radius;

    setArea (Math.PI*radius*radius);
}

/** Return String representation. */
public String toString () {
    return "[Circle (" + radius + ") area=" + getArea() + "]";
}
}

Sample Exam question(s)

1. There are three code samples. Which show evidence of overloading, overriding, polymorphism, static binding