Working Effectively with Legacy Code (draft)

Working Effectively with Legacy Code written by Michael Feathers Something I understand about the book

Contents
The Book
Definition of Legacy Code
Problems of the Conservative Approach
The Automatic Test Approach
Example of Characterization Test
Breaking Dependencies (not yet ready)
Seams (not yet ready)
Conclusion: the complaints/solutions matrix

The Book
Robert C. Martin Series
Michael C. Feathers
http://www.amazon.com/Working-Effectively-Legacy-Michael-Feathers/dp/0131177052 Paperback: 456 pages Publisher: Prentice Hall PTR; 1 edition (October 2, 2004)

What is legacy code? (for me/us?)
Code that:
is difficult to understand
is fragile
is difficult to modify
is not (or not well) documented
has been inherited from others
But that :
is still useful!

What is legacy code? (M.Feathers)
M.Feathers says:
Legacy Code ≡ Code without automatic tests
Why?

The conservative approach
No automatic tests
leads to
Manual testing
that leads to
High risk of introduce bugs
that leads to
Fear of change
so
You prefer add more mess instead cleaning up the existing code
and
the code get worse,
the cost/time of adding features increases
If it ain't broke don't fix it

Preserving Behaviour with Characterization Tests
Write tests for an existings program
Write a test that “Describe” the actual behaviour
Use these test as regression tests during the refactoring/feature addition.
Please note that: “actual” is not always the same as “correct”.

Characterization Tests Example (1/5)
public static int[] slaDjcl(double djm)
{
long ld, jd, n4, nd10;
if ((djm <= -2395520.0) || (djm >= 1e9))
{
throw new IllegalArgumentException("MJD out of valid range");
}
ld = (long) djm;
jd = ld + 2400001L;
n4 = 4L * (jd + ((6L * ((4L * jd - 17918L) / 146097L)) / 4L + 1L) / 2L - 37L);
nd10 = 10L * (((n4 - 237L) % 1461L) / 4L) + 5L;
int[] ret = new int[3];
ret[0] = (int) (n4 / 1461L - 4712L);
ret[1] = (int) (((nd10 / 306L + 2L) % 12L) + 1L);
ret[2] = (int) ((nd10 % 306L) / 10L + 1L);
return ret;
}

@Test
public void testSlaDjcl() {
assertArrayEquals(new int[]{},slaDjcl(0.0));
}
Testcase: testSlaDjcl(prova.ProvaTest): FAILED array lengths differed, expected.length=0 actual.length=3 junit.framework.AssertionFailedError: array lengths differed, expected.length=0 actual.length=3 at prova.ProvaTest.testSlaDjcl(ProvaTest.java:19)

@Test
assertArrayEquals(new int[]{0,0,0},slaDjcl(0.0));
}
Testcase: testSlaDjcl(prova.ProvaTest): Caused an ERROR arrays first differed at element [0]; expected:<0> but was:<1858> arrays first differed at element [0]; expected:<0> but was:<1858> at prova.ProvaTest.testSlaDjcl(ProvaTest.java:19)

@Test
}
BUILD SUCCESSFUL (total time: 0 seconds)

@Test
assertArrayEquals(new int[]{-3617, 1, 24},slaDjcl(-2000000.0));
assertArrayEquals(new int[]{10346, 5, 23},slaDjcl(3100000.0));
assertArrayEquals(new int[]{24309, 9, 19},slaDjcl(8200000.0));
assertArrayEquals(new int[]{38273, 1, 15},slaDjcl(1.33E7));
}

The TDD algorithm
0. Get the class you want to change under test.
1. Write a failing test case.
2. Get it to compile.
3. Make it pass. (Try not to change existing code as you do this.)
4. Remove duplication.
5. Repeat.

Conclusions: Complaints / Solutions matrix So then you ... But legacy code… add them (as characterization tests) lacks of test redesign it (with refactoring) is ugly

Conclusions: I can’t put code under tests because So you I can’t because code
Break Depedencies
Use the seams
depends of everything
Introduce sensing variabiles
Use exract method (safe version)
lacks of modularity

Thanks
Andrea Francia
http://andrefrancia.it/
http://blog.andreafrancia.it
[email_address]
These slide will be available at my blog.

Working Effectively with Legacy Code (draft)

by Andrea Francia on Nov 30, 2009

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