Homework 2 - Designing Classes

CS 2102 - Dterm 10

Homework 2 - Designing Classes with the Design Recipe

Due: Friday, March 26 at 5pm

Acknowledgement: This assignment is a modified version of an assignment designed by Dr. Viera Proulx.

This assignment and all subsequent assignments are to be done with your homework partner.

Assignment Goals

To use the design recipe to design class hierarchies
To use the design recipe to design methods
To obtain some experience using the TeachScheme/ReachJava libraries for drawing on a Canvas

Preliminaries

In this homework you will be displaying shapes on a Canvas. You will need to download the library files that allow you to draw, specify colors, and use the Posn and Canvas classes. Here's the link to the libraries. Go to the Downloads link, and from there save colors.jar, geometry.jar, and draw.jar in your EclipseJars folder (on your toaster drive or your own computer). When you create your Eclipse project you will need to add each of these libraries as "external jars" along with the tester library.

You can find documentation for the classes provided by these libraries here. Follow the links from the "draw" and "colors" libraries to the Javadocs (Java documentation) for draw, colors, and geometry. You will need import statements for any .java file that refers to a class that is defined in one of the libraries. Here are the import statements you will be using:

import colors.*;     // when you need to use an IColor
import geometry.*;   // when you need to use a Posn
import draw.*;       // when you need to use a draw method, or a Canvas
import tester.*;     // when you need to use checkExpect, checkInexact, or Tester

Restrictions (and why)

For this homework, and for all remaining labs and homework assignments, you will be restricted to using only those features of the Java language that we have previously covered during the lectures. For Homework 2, the only kinds of statements you may use in your method implementations are the if and return statements. In particular, you are not allowed to use any assignment statements (i.e. the "=" operator) except as they have been used in constructors. Also, you are not allowed to use any of Java's type-checking mechanisms - we will cover those at a later time, when their use is required.

There are a couple of reasons why we are restricting your use of Java in these ways. First, we want you to understand Java's dynamic-dispatch mechanism (polymorphism) which is at the heart of object-oriented programming. Let Java figure out types of objects for you! Secondly, we want to avoid assignment statements for the time being, because (as we began to discover in CS 1101) assignments introduce side-effects, making testing more difficult. When we get to problems that can't be solved any other way, we will then use assignment.

Problems

Design a class hierarchy to represent shapes that may be drawn on a Canvas. Here are the shapes you should define:
- A circle has the position of the center, the radius, and a color.
- A disk has the position of the center, the radius, and a color.
- A rectangle has the position of the of the upper left (northwest) corner, the height and the width, and a color.
- A line has the position of the start and end points and a color.
- A combo consists of two shapes, the top and the bottom.
Draw a class diagram of a class hierarchy that represent shapes. You won't be submitting it, but you should always start by drawing a class diagram. If you come for office hour help, we will ask you to show us your diagram.
Define a class CartPt to represent a Cartesian point with integer coordinates.
Define the classes that represent the various shapes. Use the CartPt class to represent the positions of shapes. Use the IColor interface to represent the colors of the shapes.
Make examples of data.
In this part of the homework you will use the Design Recipe for Methods* to design methods for your Shape classes. Make sure you provide test cases in the Examples class as you develop each method.
- Design the method toPosn that produces an instance of the class Posn from this CartPt. The Posn class is defined in the geometry library.
- Design the method distanceTo() that computes the distance (a double) from this CartPt to the given CartPt. You may use the Java library method that computes a square root of a number. To compute the square root of a number x, you would use the expression
```
Math.sqrt(x)
```
- Design the method contains() that determines whether this shape contains the given CartPt. (Note: To do this correctly for the class Line requires a bit of geometrical computation. You are allowed to substitute, in this case, a method that produces false for all inputs.)
- Design the method draw() that draws this shape on the given Canvas (refer to the documentation in the draw library on Canvas). You do not need to provide test cases for the draw() method. Instead, you can display shapes on a Canvas by providing statements such as the following in your Examples class:
```
  Canvas c = new Canvas(100, 100);

  boolean showShape = this.c.show() && this.myShape.draw(this.c);
```
  where myShape is an example of a shape that has been defined in the Examples class.
*Design Recipe for Methods
- Formulate a purpose statement and a method header
- Illustrate the purpose statement with functional examples (test cases)
- Lay out what you know about the method's argument(s) (including this) in a template
- Define the method
- Run your test cases

What to Turn In

Using web-based turnin, turn in a single file containing all code and documentation for this assignment. Follow the naming conventions when naming your file. Both partners' names and wpi login names should appear in a comment at the top of the file.