The document describes the OCP Kata process for practicing object-oriented design principles. It involves: 1. Writing a failing test case 2. Creating a factory to pass the test with minimal code 3. Writing another failing test and refactoring to pass it 4. Repeating steps 2-3, refactoring each time to keep tests failing until the new case is implemented An example FizzBuzz kata is provided, starting with a test for single numbers then refactoring to add logic for multiples of 3 (Fizz) through repeated tests and refactoring. The goal is to practice separation of concerns through object oriented patterns.

1. Xpug Coding Dojo:
The OCP Kata

2. Kata

3. Pairs

4. Randori

5. Costraints

6. The OCP Kata
http://matteo.vaccari.name/blog/archives/293

7. 0. Write the first failing test.

8. Then write a factory that returns an object,
or an aggregate of objects, that make the
test pass.

9. The factory should be limited to
creating objects and linking them
together. No conditionals allowed.

10. 1. Write the next failing test.

11. 2. Can you make it pass by changing the
factory and/or creating a new class and
nothing else?

12. If yes, great! Go back to 1.

13. If not, refactor until you can.

14. The refactoring should bring the
system to a state where it’s possible
to implement the next test just by
changing the aggregate of objects
that is returned by the factory.

15. Be careful not to implement new
functionality; the current test should
still fail.

16. An example

17. package xpug;
import junit.framework.Assert;
import org.junit.Test;
public class AFizzbuzzer {
@Test
public void shouldReturnSameNumberForPlainNumber() {
Fizzbuzzer fizzbuzzer = FizzbuzzerFactory.create();
Assert.assertEquals("1", fizzbuzzer.say(1));
Assert.assertEquals("2", fizzbuzzer.say(2));
}
}

18. package xpug;
public class FizzbuzzerFactory {
public static Fizzbuzzer create() {
return new Fizzbuzzer();
}
}

19. package xpug;
public class Fizzbuzzer {
public String say(int digit){
return String.valueOf(digit);
}
}

20. @Test
public void shouldReturnFizzForMultipleOfThree(){
Fizzbuzzer fizzbuzzer = FizzbuzzerFactory.create();
Assert.assertEquals("Fizz", fizzbuzzer.say(3));
Assert.assertEquals("Fizz", fizzbuzzer.say(6));
Assert.assertEquals("Fizz", fizzbuzzer.say(9));
}

21. package xpug;
public interface Counter {
public boolean canHandle(int digit);
public String say(int digit);
}

22. package xpug;
public class Echo implements Counter {
@Override
public boolean canHandle(int digit) {
return true;
}
@Override
public String say(int digit) {
return String.valueOf(digit);
}
}

23. package xpug;
import java.util.List;
public class Fizzbuzzer {
private List<? extends Counter> counters;
public Fizzbuzzer(List<? extends Counter> counters) {
this.counters = counters;
}
public String say(int digit) {
for (Counter counter : counters)
if (counter.canHandle(digit))
return counter.say(digit);
return "";
}
}

24. package xpug;
import java.util.Arrays;
public class FizzbuzzerFactory {
public static Fizzbuzzer create() {
return new Fizzbuzzer(Arrays.asList(
new Echo()));
}
}

25. @Test
public void shouldReturnFizzForMultipleOfThree(){
Fizzbuzzer fizzbuzzer = FizzbuzzerFactory.create();
Assert.assertEquals("Fizz", fizzbuzzer.say(3));
Assert.assertEquals("Fizz", fizzbuzzer.say(6));
Assert.assertEquals("Fizz", fizzbuzzer.say(9));
}

26. package xpug;
public class Fizz implements Counter {
@Override
public boolean canHandle(int digit) {
return digit % 3 == 0;
}
@Override
public String say(int digit) {
return "Fizz";
}
}

27. package xpug;
import java.util.Arrays;
public class FizzbuzzerFactory {
public static Fizzbuzzer create() {
return new Fizzbuzzer(Arrays.asList(
new Fizz(),
new Echo()));
}
}

28. ...
...
...
...
...
...
...

29. package xpug;
import java.util.Arrays;
public class FizzbuzzerFactory {
public static Fizzbuzzer create() {
return new Fizzbuzzer(Arrays.asList(
new Fizzbuzz(),
new Buzz(),
new Fizz(),
new Echo()));
}
}

30. KataYahtzee

32. Let’s start!