©2008 Felleisen, Proulx, et. al.

8  Assignment

Portfolios: Abstracting over the Datatype: Generics

1. As usual, finish the Lab 8.

2. Recall the class Song from Lab 6. Make three examples of lists of songs as instances of AList<Song>, each containing at least four songs. Now define any classes you may need so that you can use the methods defined for AList<T> to produce each of the following:

   • Does the list contain only songs that cost less than 10.00?

   • Doeas the list contains a song by "Beatles"?

   • Produce a list of those songs from this list that cost less than some given number.

   • Produce from this list a list of all songs by a given artist.

Abstracting with Interfaces and Function Objects Abstracting with Generics: Type Parameters

8.1  Problem

You will start this assignment with the given code. We will deal with two classes of data, Balloon and City. The goal will be to design common methods that work for both classes without changes.

Here is a brief inventory of the classes that are provided:

• City — the information includes the name, state, zip code, latitude, and longitude

• Balloon — the information includes the x and y coordinates of the center, the radius, and the color of the balloon

• AList<T>, MTList<T>, and ConsList<T> that you have seen already in the lab. It includes the filter method that consumes an ISelect<T> object and produces a list of all elements from this list that satisfy the given predicate.

• ISelect<T> interface that represents a select predicate

• Examples — the class with some examples of data and some tests already implemented

1. Design the following classes that implement the ISelect interface:

   • RedBalloon — that selects only the red balloons

   • SmallBalloon — that selects all balloons with the radius smaller that the value given to the constructor.

   • Below40th — that selects only the cities that are below 40th parallel of latitude

   • InState — that selects only the cities in the given state.

   Make sure you test all these classes.

2. Design and run tests for the method filter in the classes that represent a list of <T> by using all four classes that implement the ISelect<T> interface.

3. Add the following interface to your project:

   interface ShowMe<T>{
     public void display(T t);
   }
   

   Now design the following classes that implement the ShowMe interface:

   • PaintBalloon that paints the balloon data in the Canvas

   • PaintCity that paints the city as a small circle in the Canvas.

     Hint: To be able to draw the city on a Canvas, assume that the latitude and longitude lines are parallel, and that the entire map is drawn between the 65th and 125th longitude line and between 25th and 55th parallels (latitude lines).

4. Now design the method showAll in the classes that represent a list of <T>. Test is by using all four classes that implement the ShowMe interface.

5. Java Collections Framework — the libraries we will soon use — provides the following interface:

   public interface Comparator<T>{
     /* produce int < 0 if op1 is before op 2
      * produce 0 if op1 is the same as op2
      * produce int > 0 if op1 is after op2
      * in your desired ordering
      */
     public int compare(T op1, T op2);
   }
   

   Design the following classes that implement the Comparator<T> interface with methods that perform the following comparisons:

   • B1HigherThanB2 that determines whether balloon-1 is closer to the top than balloon-2

   • B1SmallerThanB2 that determines whether balloon-1 has smaller radius than balloon-2

   • C1BeforeC2 that determines whether the name of the city-1 is lexicographically before the name of the city-2

   • C1StateBeforeC2 that determines whether the state of the city-1 is lexicographically before the name of the city-2

6. Design the sort method for the classes that represent a list of <T> using the given instance of the Comparator. Test your program (and all helper methods) using all four of the classes defined above.

8.2  Problem - Extra Credit

You can now generalize the graph traversal solution from Assignment 7 to draw a graph of state capitals with lines connecting neighboring states (and all Four Corners states). Try to generalize your solution so you would be able to draw the map and show the path from one city to another.

You can hand in a text document that describes what you have tried to do and how far did you get, if you do not complete the whole program.

We suggest that you first work only with the New England states, to make the data entry less painful. Soon, we will read data like this as an input from a file.