Coupling Basics : Concrete Class vs Abstract Messages

Objective


To learn about depending on abstract messages instead of concrete classes to create re-usable code.

Steps


Step 1

Create copy.rb:

while line = gets
  puts line
end

Step 2

Run it.

$ruby copy.rb

Here is a sample run:

Hi
Hi
Hello
Hello

Ctrl+D to quit the program. This reads from keyboard and writes to the console.

Step 3

Create a readme.txt file:

This is the first line
This is the second line

Now run the program as follows:

$ruby copy.rb readme.txt

This reads the readme.txt from the file and writes it to the console.

Step 4

Create file_copy.rb:

File.open('./readme.txt', 'r') do |file|
  while line = file.gets
    puts line
  end
end
$ruby file_copy.rb

This reads the readme.txt and writes it to the console.

Step 5

Create abstract.rb with:

require 'stringio'

ip = StringIO.new('This is a test')
op = StringIO.new('', 'w')

ip.each_line do |line|
  op.puts line
end

print op.string

Step 6

This uses StringIO a fake file system to read and write. This is good for testing. The dependency is on the message: each_line and puts and not on any concrete class. If there is a name of the class in the code, it creates tight coupling in the code and makes it difficult to reuse. As long as the ip and op variables can respond to the each_line and puts messages. This program will work.

Depend on messages that capture abstractions which can be varied by having different implementations.

Summary


In this article, we saw examples for:

Standard In --> Standard Out
Keyboard  --> Console
File      --> Console
Fake File --> Fake Console

You learned that depending on a concrete class creates tight coupling and depending on messages that can be implemented by different implementations in different class can lead to re-usable code.


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