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Practical Object Oriented Design with Ruby

Part 1: Designing Classes with a Single Responsibility

First, read chapters 1 and 2 (through page 34).

Once you finish, think about a commercial airplane. Create a list of at least 15 classes you'd expect to encounter to build a software model of a plane. For each class, give a one sentence description of the class' responsibility

Part 2: Experimenting with Dependencies

Read a part of Chapter 3, pages 35 through 42 up to "Isolate Dependencies".

Then, let's do an experiment with dependency injection. Check out this usage:

screen_printer = ScreenPrinter.new
fibber = Fibber.new(screen_printer)
fibber.print(10) # => prints the first 10 numbers of the fibonacci sequence to
                 #    the screen separated by a single space
file_printer = FilePrinter.new("output.txt")
fibber = Fibber.new(file_printer)
fibber.print(10) # => prints the first 10 number of the fibonacci sequence to
                 #    a file named 'output.txt' separated by a single space

Build Fibber, ScreenPrinter, and FilePrinter classes to make that work.

Part 3: Experimenting with Arguments

Read pages 42-46 just for completeness. Then read 46-51 focusing on argument ordering and defaults.

Imagine that you're writing a real estate application. We need to create records (objects) that represent both a SingleFamilyHome and an ApartmentHome. Both of them have...

  • A square_footage. For a SingleFamilyHome the default is 1000. For an ApartmentHome the default is 400
  • A monthly_payment. For a SingleFamilyHome the default is 1500. For an ApartmentHome the default is 800

Build an implementation of these two classes. Does it make sense for them to both inherit from a single ancestor?

In addition to their common attributes...

  • A SingleFamilyHome has a boolean has_yard attribute
  • A SingleFamilyHome has a year_built attribute
  • An ApartmentHome has a maintainance_fee attribute
  • An ApartmentHome has a boolean doorman attribute

Can you implement these two classes such that all attributes can be set in the initializer, but only the square_footage, monthly_payment, and year_built/maintainance_fee are required?

Part 4: Further Dependency Considerations

Read pages 51 through the end of the chapter on 57 and answer the following two questions:

  1. Look at the Enigma project you've completed. What are two or three components or ideas in the system that are likely to change? Does your implementation make those easy or difficult to change?
  2. Consider the character map in Enigma. Imagine that there are multiple character maps available for messages. Maybe some messages are in all normal ASCII characters, while others use cyrillic letters. The user might specify the character set by name on the command line. How would you tweak your implementation to handle multiple character sets at once?