„Working Effectively with Legacy Code“- one of the best books which talks about Unit testing and TDD. Presentation covers best practices listed in the book as well as shows which of them were successfully implemented in practice. You will find out how to commit changes and be sure that they work even without running and testing application itself.
A unit test that takes 1/10th of a second to run is a slow unit test. If you have a project with 3,000 classes and there are about 10 tests apiece, that is 30,000 tests. How long will it take to run all of the tests for that project if they take 1/10th of a second apiece? Close to an hour.
Elaborate more on problem localization
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Lag time is the amount of time that passes between a change that you make and the moment that you get real feedback about the change
Generally, when we want to get tests in place, there are two reasons to break dependencies: sensing and separation.
One of the most valuable things about TDD is that it lets us concentrate on one thing at a time. We are either writing code or refactoring; we are never doing both at once. That separation is particularly valuable in legacy code because it lets us write new code independently of new code. After we have written some new code, we can refactor to remove any duplication between it and the old code.
Usually unexpected behavior IS expected from the whole application!
If the system has been deployed, you need to examine the possibility that someone is depending on that behavior, even though you see it as a bug.
Know your code better!
Best way to get familiar with the code!
Bye Bye Cowboy Coder Days! (Legacy Code & TDD)
Bye Bye Cowboy-Coder
What do developers do for living?
Change the code!
Four Reasons to Change Software
• Adding a feature
• Fixing a bug
• Improving the design
• Optimizing resource usage
Fixing a Bug Refactoring Optimizing
Structure Changes Changes Changes
Existing Behavior New Behavior
1. What changes do we have to make?
2. How will we know that we've done them correctly?
3. How will we know that we haven't broken anything?
Time to fix/Price
1 Minute 10 Minutes 1 Hour 1 Day 1 Week
Time to fix/Price
The Legacy Code Dilemma
When we change code, we should have tests in place. To put
tests in place, we often have to change code.
The Legacy Code Change Algorithm
1. Identify change points
2. Find test points
3. Break dependencies
4. Write tests
5. Make changes and refactor
Sensing and Separation
• We break dependencies to sense when we can't access
values our code computes
• We break dependencies to separate when we can't even get
a piece of code into a test harness to run
1. Identify where to change
2. Wdown a method call and then comment it out.
3. Determine what local variables you need for the call
4. Determine return value
5. Develop the sprout method using test-driven development
6. Remove the comment in the source method
1. Identify where you need to make your code change.
2. Write class in that place, and call a method; then
comment those lines out
3. Determine what local variables you need from the source
method, and make them arguments to the classes'
4. Determine return values to the source method and add a
call in the source method to receive those values.
5. Develop the sprout class test first
6. Remove the comment in the source
1. Identify a method you need to change
2. If the change can be formulated as a single sequence of
statements in one place, develop a new method for it
using test-driven development
3. Create another method that calls the new method and the
1. Identify a method where you need to make a change.
2. Create a class that accepts the class you are going to
wrap as a constructor argument.
3. Create a method on that class, using test-driven
4. Write another method that calls the new method and the
old method on the wrapped class
5. Instantiate the wrapper class in your code in the place
where you need to enable the new behavior
TDD and Legacy Code
1. Get the class you want to change under test
2. Write a failing test case
3. Get it to compile
4. Make it pass (Try not to change existing code as you do
5. Remove duplication
Allways check if tests are testing
1. Write test and new code in separate changelist
2. Shelve new code change list
3. Ensure tests are failing
1. Use a piece of code in a test harness
2. Write an assertion that you know will fail - let the
failure tell you what the behavior is
3. Change the test so that it expects the behavior that
the code produces
I Can't Get This Class into a Test
• Lean on the Compiler
• Extract Interface
• Extract Implementer
• Subclass and Override Method
• Parameterize Constructor
• Parameterize Method
• Extract and Override Getter
• Extract and Override Factory Method
• Singleton Design Pattern
When All Else Fails, Do Some Scratch