8.7. Studio: Spinning the Discs

Although they look the same, the old optical discs—CDs and DVDs—are quite different from each other. Let’s use them as the focus for creating a new interface.

8.7.1. Getting Ready

Set up a local copy of the project:

  1. Visit the repository page for this project and fork it to create a copy under your own GitHub account.
  2. Open up the solution in Visual Studio to get started.

8.7.2. Disc Project Overview

In this activity, you will pair up to design a new custom interface and an abstract class.

Take a look at the prepared CD and DVD classes in the starter code. Neither class contains members and methods. You will add these by extending a base class and implementing your interface. Before you do this, however, you need to consider what code to place in the interface vs. the abstract class vs. each specific class.

Just like you did with your Restaurant Menu studios, you will begin with pen and paper (or similar tools).

8.7.3. Consider Generic Behaviors

Here are a few behaviors that both CDs and DVDs share:

  1. They both spin.
  2. They both store data.
  3. Data gets written to the discs by using a laser.
  4. The data on the discs can be read by using a laser.
  5. Once loaded, they both report information like name, capacity, contents, and disc type.

With your partner, add one or two more behaviors to the list. Feel free to do some quick research online if needed. Note that information like storage capacity does NOT belong on this list, since it describes a property rather than a behavior.

For each behavior, identify if it depends on any type of instance variable. For example, the spinning behavior does not require any field information from the CD or DVD classes.

This collecting and sorting of behaviors is the first step toward designing your interface and abstract class. It prompts you to consider behaviors that can be generally applied vs. those that depend on data stored in a specific object.

8.7.4. Consider Class-Specific Behavior and Data

Next, you and your partner need to decide which behaviors go into the interface vs. an abstract class.

Behaviors added to an interface do NOT have to be identical for every object that implements the interface. The key is that the methods require no object-specific data. The methods can accept parameters, but these should represent generic values like a search term rather than field data like the title of a DVD.

For example, even though CDs and DVDs both spin, they do so at different speeds. Their behavior (spinning) is the same, but the details of that behavior vary. Thus, a method like SpinDisc() fits well into an interface.

  1. For the other behaviors you and your partner sorted, decide which ones belong in the interface.
  2. Write down details about how these behaviors differ for CDs and DVDs. This guides your thinking about how each class should implement the methods.
  3. For the behaviors that cannot go into the interface, decide if they could be placed in an abstract class or if they need to appear in the separate CD and DVD classes.
  4. Finally, write down the fields and properties needed to define the properties of a CD or DVD (e.g. StorageCapacity). Consider:
    1. Which members are shared by both types of discs, and which ones are unique to each class?
    2. Do any of the members represent constants?

8.7.5. Code the Interface

Now that your team has completed the before-coding thinking, it’s time to start building!

  1. Add an IOpticalDisc file for the interface. Refer back to Creating an Interface if you need a quick review of the syntax.

  2. Declare and initialize any constants.

  3. Add a method signature for each of the behaviors you identified for the interface. For example:

    void SpinDisc();

Good work. Now you need to step back from the interface for a moment and take care of the remaining methods and fields.

8.7.6. Code the Abstract Class

To streamline the field and constructor code for CD and DVD, you need to start with a base class.

  1. Create an abstract BaseDisc class and declare the fields used by both CD and DVD.
  2. Code the shared constructors and required methods.

OK. Now put your code to work.

8.7.7. Complete the Disc Classes

  1. Extend the base class into the CD and DVD classes.
  2. Implement the interface in each class.
  3. Fill in the code for each of the interface methods.
  4. Add any other necessary methods needed by each class.

8.7.8. Add Method Calls to Main

In the Main method, declare and initialize a CD and DVD object, then run each of the behaviors you and your partner identified.




A CD spins at a rate of 200 - 500 rpm.
A DVD spins at a rate of 570 - 1600 rpm.

8.7.9. Bonus Missions: Other Discs & Disks?

  1. CDs and DVDs are not the only flat, circular media that have been used to store and return data.
    1. Add classes for vinyl records (LPs) and floppy disks.
    2. Does your interface apply to all four classes? If so, implement the interface and its methods in the LP and floppy classes.
    3. If your interface does NOT apply to all of the classes, modify it to do so. (Note: At the very least, you need to rename the interface).
    4. Does your abstract class apply to LPs and floppy disks? If so, extend it into the new classes. If not, should you modify it or extend a different abstract class?
  2. What about wheels and Frisbees? These are also spinning disks, but they are not used for data storage.
    1. Abstract class or interface? Which one can be applied to the Wheel and Frisbee classes?
    2. Discuss these questions with your peers, then code one of the classes just for fun.