MP9

Homework 9: Refactoring

In this homework you will apply patterns to refactor the method retrieveContents: in the FtpUrl class. This method retrieves the contents of FTP sites. It is long and not easy to read. Before making any changes to it, make a reference copy of this method - it may save you frustrations later on.

As usual, a test file is provided at the bottom of this page. File it in. Run it after every step to make sure that you don't introduce any bugs. Read the setUp method before you start to understand what the tests do.

In your refactoring, follow these steps in order:

1) Apply the Method Object pattern. Create a class for the retrieveContents: method. Move the method to the new class. Change method's local variables into instance variables of the new class. Change the original method in FtpUrl class so that it creates the Method Object and calls it. Test to make sure that the code works. (Remember to change the references to 'self' when you move the method).

2) Using the Extract Method refactoring in the Refactoring Browser, break the original method into smaller methods. Give each new method a meaningful name. Leave the code starting with "filename == nil ifFalse" as a single method for now. If an instance variable is used in only one method, make it temporary to that method. Test to make sure that the code still works.

3) Note that a large portion of the original method depends on the URL format. Requests for files are handled differently then requests for directories. The code at the beginning of the original method determineswhether the URL is a file or a directory and the code at the end of the method performs different processing based on this distinction.
Use the Choosing Message pattern to refactor this part of the code. Create subclasses of your Method Object, one for files, and another one for directories. Have the retrieveContents method in the FtpUrl class invoke a new class method on your superclass. In that method, instantiate the appropriate subclass (i.e. move the code from the beginning of the original method to the new class method). In the subclasses, implement a method with the file- or directory-specific processing. Define it as an abstract method in the superclass. Test again. And again :) Note that making the test cases green does not mean that your code works, you need to make sure that the Scamper shows meaningful data.

4) Finally, clean up the code to ensure that each variable is defined only in the classes/methods that use it.

Look back the results. Isn't this code easier to understand than the original?

Advice

FtpUrlRefactoringTest>>testFTPFileNoScamper

	| url content |

	url := FtpUrl new schemeName: 'ftp' authority: 'ftp.freebsd.org' path: #('pub' 'FreeBSD' 'README.TXT') query: ''.

	content := url retrieveContents content.

	Transcript clear; show: content.

	self assert: (content size > 0). 

FtpUrlRefactoringTest>>testFTPDirectoryNoScamper

	| url content |

	url := FtpUrl new schemeName: 'ftp' authority: 'ftp.freebsd.org' path: #('pub' 'FreeBSD/') query: ''. "note the trailing slash"

	content := url retrieveContents content.

	Transcript clear; show: content.

	self assert: (content size > 0). 

Files

Deliverables

• methods that do not fit in the browsers' window


• non-descriptive method names


• method names that start with get/set


• conditional statements (use polymorphism instead)

Link to this Page

• Machine Problems last edited on 14 May 2008 at 4:25:53 pm by c-98-212-224-168.hsd1.il.comcast.net