8.6. Exercises: Interfaces and Polymorphism

As a new Java coder, it might take you some time to recognize the usefulness of interfaces. At first glance, these tools do not seem to provide much benefit over extending a base class, adding instance methods to a class, or overriding a method like toString.

To help overcome this, let’s consider a common occurrence—sorting an ArrayList of objects.

1. If the list contains String or numerical entries, then sorting the list is trivial:

   Collections.sort(arrayListName);
   

2. However, if the elements are custom objects (like Cat), then sorting the list becomes more complicated. This is because the objects may contain multiple fields, any of which could be used as a sorting option. For Cat objects, this could include name, age, or mass.

8.6.1. Getting Started

Work on these exercises in the IntelliJ java-web-dev-exercises project. You will find the starter code in the lsn7interfaces package. Go ahead and open the folder and take a quick look at the class files.

You will practice implementing interfaces by playing around with a small ice cream store. It consists of a refrigerated display Case, which contains a collection of ice cream Flavor objects and a selection of Cone objects.

Tip

Did you notice the abstract Ingredient class? This gets extended into Flavor and Cone to help streamline the code.

8.6.2. Sorting Flavors by Name

To display a menu for your customers, you need to sort the ice cream flavors alphabetically by their name field. Fortunately, the Comparator interface helps you solve the sorting-objects-by-field problem.

8.6.2.1. Create a Sorting Class

1. Create a new class called FlavorComparator and have it implement the Comparator interface:

   public class FlavorComparator implements Comparator<Flavor>
   

2. Notice that IntelliJ flags a couple of errors that you need to fix:

   1. Import java.util.Comparator. This removes the flag on Comparator.

   2. Hover over the line again and select implement methods. Choose the compare option.

      

   3. This adds an @Override method that compares two Flavor objects and always returns 0.

      @Override
      public int compare(Flavor o1, Flavor o2) {
         return 0;
      }
      

3. Always returning 0 results in no sorting, so replace line 8 with:

   return o1.getName().compareTo(o2.getName());
   

   This returns an integer (negative, positive, or zero) depending on whether Flavor object o1 or o2 comes first, alphabetically.

Check your solution

8.6.2.2. Sorting the flavors ArrayList

In Main, we declare menu that contains everything in the Case as well as specific flavors and cones collections.

public static void main(String[] args){
   Case menu = new Case();
   ArrayList<Flavor> flavors = menu.getFlavors();
   ArrayList<Cone> cones = menu.getCones();

}

1. To sort the flavors list, first create a new FlavorComparator object.

   public static void main(String[] args){
      Case menu = new Case();
      ArrayList<Flavor> flavors = menu.getFlavors();
      ArrayList<Cone> cones = menu.getCones();
      Comparator comparator = new FlavorComparator();
   }
   

2. Next, call the sort method on flavors and pass the comparator object as the argument.

   public static void main(String[] args){
      Case menu = new Case();
      ArrayList<Flavor> flavors = menu.getFlavors();
      ArrayList<Cone> cones = menu.getCones();
      Comparator comparator = new FlavorComparator();
   
      flavors.sort(comparator);
   }
   

3. Iterating through the flavors list before and after the sort shows the results. (The output below displays just the name fields).

   Before:                 After:
   
   Vanilla                 Chocolate
   Chocolate               Red Velvet
   Red Velvet              Rocky Road
   Rocky Road              Strawberry Sorbet
   Strawberry Sorbet       Vanilla
   

Note that Main does NOT have to implement the Comparator interface. This only needs to happen in the class that actually uses the compare method.

Tip

Instead of declaring and initializing the comparator object, we could combine steps 1 and 2 by using a single statement:

flavors.sort(new FlavorComparator());

8.6.3. Sorting Cones by Cost

Now let’s sort our cones list by cost, from least expensive to most expensive.

1. Create the new class ConeComparator.

2. Follow the example above to implement the Comparator interface and evaluate Cone objects by cost.

3. In Main, sort the cones list, then print the elements to the screen to verify the results.

   Before:           After:
   
   Waffle: $1.25        Bowl: $0.05
   Sugar: $0.75         Wafer: $0.50
   Wafer: $0.50         Sugar: $0.75
   Bowl: $0.05          Waffle: $1.25
   

Check your solution

8.6.3.1. Troubleshooting

Did you get this error?

This happens because according to the interface, compare MUST return an integer value, but the cost fields are double type.

To fix this, use an if/else if/else block to evaluate o1.getCost() - o2.getCost(). Return a positive integer, negative integer, or 0 depending on the result.

8.6.4. Bonus Exercises

1. Modify FlavorComparator to sort Flavor objects by the number of allergens, from highest to lowest.

2. Create a Topping class that extends Ingredient. Add toppings to the Case constructor, then choose how to sort a toppings array in Main.

8.6.5. Next Steps

In these exercises, you practiced implementing existing interfaces. In the studio activity, you will design and implement your own.