9.9.3. Trio Student Solution 2

The following is a free response question from 2014. It was question 4 on the exam. You can see all the free response questions from past exams at https://apstudents.collegeboard.org/courses/ap-computer-science-a/free-response-questions-by-year.

Question 4. The menu at a lunch counter includes a variety of sandwiches, salads, and drinks. The menu also allows a customer to create a “trio,” which consists of three menu items: a sandwich, a salad, and a drink. The price of the trio is the sum of the two highest-priced menu items in the trio; one item with the lowest price is free. Each menu item has a name and a price. The four types of menu items are represented by the four classes Sandwich, Salad, Drink, and Trio. All four classes implement the following MenuItem interface.

public interface MenuItem
{
    /**
     * @return the name of the menu item
     */
    String getName();

    /**
     * @return the price of the menu item
     */
    double getPrice();
}

The following diagram shows the relationship between the MenuItem interface and the Sandwich, Salad, Drink, and Trio classes.

Figure 1: UML class diagram showing MenuItem is implemented by Sandwich, Salad, Drink, and Trio.

For example, assume that the menu includes the following items. The objects listed under each heading are instances of the class indicated by the heading.

Figure 2: Example objects

The menu allows customers to create Trio menu items, each of which includes a sandwich, a salad, and a drink. The name of the Trio consists of the names of the sandwich, salad, and drink, in that order, each separated by “/” and followed by a space and then “Trio”. The price of the Trio is the sum of the two highest-priced items in the Trio; one item with the lowest price is free. A trio consisting of a cheeseburger, spinach salad, and an orange soda would have the name "Cheeseburger/Spinach Salad/Orange Soda Trio" and a price of $4.00 (the two highest prices are $2.75 and $1.25). Similarly, a trio consisting of a club sandwich, coleslaw, and a cappuccino would have the name "Club Sandwich/Coleslaw/Cappuccino Trio" and a price of $6.25 (the two highest prices are $2.75 and $3.50).

9.9.3.1. Grading Rubric

Below is the grading rubric for the Trio class problem.

Figure 3: The grading rubric for the Trio class problem.

9.9.3.2. Practice Grading

The following is the second sample student response.

Figure 4: The start of the second sample student response to the Trio class problem.

Figure 5: The end of the second sample student response to the Trio class problem.

Apply the grading rubric shown above as you answer the following questions.

Apply the Grading Rubric

9-9-2-1: Should the student earn 1 point for the correct declaration of the Trio class?

• Yes
• This declares the class correctly as public class Trio implements MenuItem
• No
• What do you think is wrong with the class declaration?

9-9-2-2: Should the student earn 1 point for declaring the private instance variables (sandwich, salad, and drink or name and price)?

• Yes
• Remember that all instance variables should be declared private so that the class controls access to the variables.
• No
• The student did not make the instance variables private, so the student does not get this point.

9-9-2-3: Should the student earn 1 point for declaring the the constructor correctly?

• Yes
• This solution declares the constructor as public Trio(Sandwich s, Salad sa, Drink d)
• No
• What do you think is wrong with the constructor declaration?

9-9-2-4: Should the student earn 1 point for correctly initializing the appropriate instance variables in the constructor?

• Yes
• This solution initializes the instance variables (sandwich, salad, and drink) correctly with the values from the parameters (s, sa, and d).
• No
• What do you think is wrong with the initialization of the instance variables in the constructor?

9-9-2-5: Should the student earn 1 point for correctly declaring the methods in the MenuItem interface (getName and getPrice)?

• Yes
• This solution contains correct declarations for public String getName() and public double getPrice().
• No
• To implement an interface the class must have a getName and getPrice method as defined by the MenuItem interface.

9-9-2-6: Should the student earn 1 point for correctly constructing the string to return from getName and making it available to be returned?

• Yes
• Look at what getName is supposed to return.
• No
• This solution doesn't include the "/" between the sandwich and salad and between the salad and the drink and is also missing the "Trio" at the end of the name, so it loses this point.

9-9-2-7: Should the student earn 1 point for returning a constructed string from getName?

• Yes
• This solution does return the constructed string, even if the string is not completely correct.
• No
• Even though the string is not correct it was constructed and returned.

9-9-2-8: Should the student earn 1 point for correctly calculating the price and making it available to be returned from getPrice?

• Yes
• What if b is equal to c but both are greater than a?
• No
• This does not always compute the price correctly (when b is equal to c and they are both greater than a, it should return b+c, not a+b).

9-9-2-9: Should the student earn 1 point for returning the calculated price in getPrice?

• Yes
• This solution does return the calculated price, even if that price is not always correct.
• No
• This point is earned if the student attempted to calculate the price and returned what was calculated.

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