On this page:
2.1 Guided example
2.1.1 Initial scenario
2.1.2 Solution:   delegation
2.2 Methods for Structured Data
7.2

Lab 2: Working with Self-Referential Data

Goals: The goals of this lab are to review data definitions and practice designing self-referential classes and data, then design methods for a variety of class herarchies.

Related files:
  tester.jar     javalib.jar  

For this lab, there are no starter files. For each problem, start a new project and build the files from scratch.

2.1 Guided example

This section provides a guided walkthrough of translating Fundies 1-style data definitions into Java.

2.1.1 Initial scenario

Suppose we want to represent people and their modes of transportation, namely bicycles and cars. First, come up with some examples:

One way to represent this information is as follows:

;; A MOT (ModeOfTransportation) is one of
;; -- Bicycle
;; -- Car
 
;; A Bicycle is a (make-bicycle String)
(define-struct bicycle (brand))
 
;; A Car is a (make-car String Number)
(define-struct car (make mpg))
 
;; A Person is a (make-person String MOT)
(define-struct person (name mot))
 
(define diamondback (make-bicycle "Diamondback"))
(define toyota (make-car "Toyota" 30))
(define lamborghini (make-car "Lamborghini" 17))
 
(define bob (make-person "Bob" diamondback))
(define ben (make-person "Ben" toyota))
(define becca (make-person "Becca" lamborghini))

Following Lecture 2, we can convert these data definitions into Java:

// Represents a mode of transportation interface IMOT {}
 
// Represents a bicycle as a mode of transportation class Bicycle implements IMOT {
String brand;
 
Bicycle(String brand) {
this.brand = brand;
}
}
 
// Represents a car as a mode of transportation class Car implements IMOT {
String make;
int mpg; // represents the fuel efficiency in miles per gallon  
Car(String make, int mpg) {
this.make = make;
this.mpg = mpg;
}
}
 
// Keeps track of how a person is transported class Person {
String name;
IMOT mot;
 
Person(String name, IMOT mot) {
this.name = name;
this.mot = mot;
}
}

Then, our examples would become the following:

class ExamplesPerson {
IMOT diamondback = new Bicycle("Diamondback");
IMOT toyota = new Car("Toyota", 30);
IMOT lamborghini = new Car("Lamborghini", 17);
 
Person bob = new Person("Bob", diamondback);
Person ben = new Person("Ben", toyota);
Person becca = new Person("Becca", lamborghini);
}

Now that we have our data definitions and examples, let’s try to write a method. Consider the following purpose statement:

// Does this person's mode of transportation meet the given fuel // efficiency target (in miles per gallon)?

Do Now!

How would you design a function like this in BSL?

Following the same design recipe steps as always, we need a purpose statement (given to us), a signature, some examples, and a template. Let’s reorder these steps slightly, and first try to determine the template for this function, i.e. what data this method will work on. This will help us decide which interface or class should contain the method’s implementation. The wording “this person’s”suggests that this should be within the Person class, since it will be evaluating a person’s method of transportation.

Next, we need to figure out the signature. A mode of transportation either meets a target fuel efficiency or it doesn’t, so it would make sense for this method to return a boolean. Finally, we need to figure out this methods parameters (or if it even needs any). In this case, the method needs to be provided a target fuel efficiency, so one paramter int mpg should work. So our method header will look like the following:

// In the Person class: boolean motMeetsFuelEfficiency(int mpg) { ... }

Do Now!

Now that we have a signature and purpose, and figured out which class this method should be in, create some examples for how this method should behave.

Now it’s time to try to implement the method, and to do that we need to work thorugh our available template. In doing so, we quickly realize that we do not have enough information. The only fields inside Person are String name and IMOT mot, neither of which can be directly compared to the given input.

Do Now!

Why do we not want to simply write a getMpg method on IMOT and its implementing classes?

2.1.2 Solution: delegation

Because Person doesn’t have enough information directly available, we need to find an available object of some other interface or class, and ask it to finish the work for us and return to us the answer. We call this approach delegation. In this example, we would likely want to delegate the work to the IMOT interface because the modes of transportation are the only ones that can truly tell us whether or not they are more fuel efficient than the given target fuel efficiency. Luckily, the result we want from this “wish list” method is the same as the return type for the method we are writing. Also, this new method would need the same input as our current method in order to evaluate fuel efficiency. Therefore, we could add this method header to IMOT:

// in IMOT // returns true if this mode of transportation is at least // as efficient as the given mpg, false otherwise boolean isMoreFuelEfficientThan(int mpg);

Now, before we actually implement this method in Bicycle and Car, let’s finish the method in Person assuming that our new method will work as intended. Since we now have this method on IMOT, we can simply invoke the method and return the result:

// in Person boolean motMeetsFuelEfficiency(int mpg) {
return this.mot.isMoreFuelEfficientThan(mpg);
}

Then, we can finish up the method in Bicycle:

// in Bicycle // a bicycle does not consume fuel, so it will always be more fuel efficient boolean isMoreFuelEfficientThan(int mpg) {
return true;
}

and Car:

// in Car // compare this car's fuel efficiency to the given fuel efficiency boolean isMoreFuelEfficientThan(int mpg) {
return this.mpg >= mpg;
}

Do Now!

Write tests on our examples for a target fuel efficiency of 15 mpg. What about 25 mpg?

Exercise

How would we do this if we wanted to compare to another mode of transportation’s fuel efficiency instead of a given target fuel efficiency? More precisely, what if we wanted to change the method header in Person to the following:

boolean motIsMoreFuelEfficientThan(IMOT mot)

Where do you get stuck?

2.2 Methods for Structured Data

Here are some unfinished classes that represent pets and pet owners:

// to represent a pet owner class Person {
String name;
IPet pet;
int age;
 
Person(String name, IPet pet, int age) {
this.name = name;
this.pet = pet;
this.age = age;
}
}
// to represent a pet interface IPet { }
 
// to represent a pet cat class Cat implements IPet {
String name;
String kind;
boolean longhaired;
 
Cat(String name, String kind, boolean longhaired) {
this.name = name;
this.kind = kind;
this.longhaired = longhaired;
}
}
 
// to represent a pet dog class Dog implements IPet {
String name;
String kind;
boolean male;
 
Dog(String name, String kind, boolean male) {
this.name = name;
this.kind = kind;
this.male = male;
}
}

Follow the design recipe to design the following methods for these classes: