©2010 Felleisen, Proulx, et. al.

3  Complex Data Definitions; Designing Methods

3.1  Problem: Methods for simple classes and containment

Design the following methods for the classes that represent pets that you have defined during the previous lab:

1. Method weighsLessThan that determines whether the pet weighs less than the given weight limit for flying in the passenger cabin of an airplane. (Each airline has their own limit.)

2. Method sameOwner that tells us whether the owner of the pet is the same as the owner of the given pet. Do this for first two variants of the Pet class.

3. Method newWeight that produces a new Pet same as the original one, but with the weight changed to the new weight, as the pet visits the veterinarian.

4. Method changeOwner that produces a new Pet same as the original one, but with the owner changed to the new owner. Do this for first two variants of the Pet class.

5. Method olderOwner that determines whether the Owner of one Pet is older than the Owner of another Pet. Do this for second variant of the Pet class.

3.2  Problem: Designing Methods: Unions of Classes

In the previous lab you have designed the class hierarchy that represents the following kinds of pets:

• cats where we record whether it is a short-hair cat of a long-hair cat

• dogs where we record the breed (e.g. Husky, Labrador, etc., or Mutt — describing an unknown breed)

• gerbils where we need to know whether it is a male of female

still keeping track of the name of the animal and of its owner.

1. Design the method isAcceptable that determines whether the pet is acceptable for a child that is allergic to long haired cats, gets along only with Labrador and Husky dogs, and should not have a female gerbil pets.

2. Design the method isOwner that determines whether this animal’s owner has the given name.

3.3  Problem: Using the draw library

Learn how to draw shapes using the draw library.

1. Download the program DrawFace.java. Run it. You need to include the libraries draw.jar, colors.jar, and geometry.jar in your project, as you have done before for the funjava.jar and tester.jar.

   The program illustrates the use of the draw library that allows you to draw shapes on a Canvas. The first three lines specify that we will be using three libraries (programs that define classes for us to use). The colors library defines a union of six colors (black, white, red, yellow, blue, and green) through the interface IColor. The geometry library defines a single class Posn that has no methods besides the constructor. The draw library does the work – allows you to define a Canvas of the given size and to draw shapes on the Canvas.

   Define the class Picture that represents a simple picture that will be shown in the Canvas. The class only needs to know the current coordinates of some anchor point of the picture (its center, or its top left corner).

   Design a picture that consists of at least one of each: a circle, a disk, a rectangle, a line, and a text. Now design the method draw in the class Picture that draws this picture on the given Canvas. Assume the size of the Canvas is always 100 by 100.

2. Design the method moveWithin that produces a new Picture moved by the given dx and dy, but using a wrap-around, i.e, if the picture would disappear to the left, it will re-emerge on the right, etc.

3. Design the method onKey that consumes a String and produces a new Picture moved in the given direction "up", "down", "left", or "right" 3 pixels, with the same constraints as in the previous method.

3.4  Problem: Strings

For this problem start with the file Strings.java that defines a list fo Strings.

Note: The following method defined for the class String may be useful:

// does this String come before that String lexicographically?
// produce value < 0   --- if this String comes before that String
// produce value zero  --- if this and that String are the same
// produce value > 0   --- if this String comes after that String
int compareTo(String that)

1. Design the method isSorted that determines whether the list is sorted in alphabetical order.

   Hint: You may need a helper method. You may want remember to the accumulator style functions we have seen in Scheme.

2. Design the method merge that consumes two sorted lists of Strings and produces a sorted list of Strings that contains all items in both original lists (including duplicates).

Again, make sure you keep updating the TEMPLATE as you go on.

Save the work you have done.