step-by-step.md

Day 1: Fight primitive obsession...

What is the problem with this kind of code?

public String composeMessage(String reindeerName, String currentLocation, int numbersOfDaysForComingBack, int numberOfDaysBeforeChristmas) {
    var daysBeforeReturn = daysBeforeReturn(numbersOfDaysForComingBack, numberOfDaysBeforeChristmas);

    return "Dear " + reindeerName + ", please return from " + currentLocation +
            " in " + daysBeforeReturn + " day(s) to be ready and rest before Christmas.";
}

There are 4 parameters of primitive types... it can be really confusing when using this kind of code.

Indeed, even if he has written it, Santa made a mistake by probably inverting the parameters of the same type numbersOfDaysForComingBack and numberOfDaysBeforeChristmas. Primitive types are very general and could represent anything... they can not carry behaviors as well...

We may use a data structure to fight primitive obsession and represent the reindeer information here.

Primitive obsession is a code smell in which primitive data types are used excessively to represent your data models.

Introduce Reindeer Object

Our IDEs can really help us fight this code smell.

• Introduce Parameter Object refactoring can help us
  • It will create a class based on our method parameters

• Configure the class by selecting which parameters should go inside the class
  • We exclude numberOfDaysBeforeChristmas to create a cohesive abstraction of the Reindeer in this context

• Here is the impact on the code
  • The IDE has instantiated the Reindeer in the test by itself 😎

// Reindeer
public record Reindeer(String reindeerName, String currentLocation, int numbersOfDaysForComingBack) {
}

// SantaCommunicator
public String composeMessage(Reindeer reindeer, int numberOfDaysBeforeChristmas) {
    var daysBeforeReturn = daysBeforeReturn(reindeer.numbersOfDaysForComingBack(), numberOfDaysBeforeChristmas);

    return "Dear " + reindeer.reindeerName() + ", please return from " + reindeer.currentLocation() +
            " in " + daysBeforeReturn + " day(s) to be ready and rest before Christmas.";
}

// SantaCommunicatorTest
@Test
void composeMessage() {
    var message = communicator.composeMessage(new Reindeer(DASHER, NORTH_POLE, 5), numberOfDayBeforeChristmas);
    assertThat(message).isEqualTo("Dear Dasher, please return from North Pole in 17 day(s) to be ready and rest before Christmas.");
}

• We do the same for the isOverdue method
  • The configuration is slightly different because the class already exists here

🔵 let's rename some fields

// No need to have a redundant reindeerName field
public record Reindeer(String name, String location, int numbersOfDaysForComingBack) {
}

What could still be improved?

public class SantaCommunicator {
    private final int numberOfDaysToRest;

    public SantaCommunicator(int numberOfDaysToRest) {
        this.numberOfDaysToRest = numberOfDaysToRest;
    }

    public String composeMessage(Reindeer reindeer, int numberOfDaysBeforeChristmas) {
      ...
    }

    public boolean isOverdue(Reindeer reindeer, int numberOfDaysBeforeChristmas, Logger logger) {
      ...
    }

    private int daysBeforeReturn(int numbersOfDaysForComingBack, int numberOfDaysBeforeChristmas) {
      ...
    }
}

int numberOfDaysBeforeChristmas is passed as an argument in each method... we may pass it to the SantaCommunicator constructor.

Configure the SantaCommunicator

We may inject numberOfDaysToRest and numberOfDaysBeforeChristmas in the constructor.

🔴 we add an expectation on the SantaCommunicator constructor from our test

🟢 we generate the constructor from our test

• we call the primary constructor

public SantaCommunicator(int numberOfDaysToRest, int numberOfDayBeforeChristmas) {
    this(numberOfDaysToRest);
}

🔵 let's use a type to represent the configuration

• We use the same refactoring technique:

• The code now looks like this:

public SantaCommunicator(int numberOfDaysToRest) {
    this.numberOfDaysToRest = numberOfDaysToRest;
}

public SantaCommunicator(Configuration configuration) {
    this(configuration.numberOfDaysToRest());
}

• We can store the Configuration in a private field
  • And use it in the methods below

public class SantaCommunicator {
    private final int numberOfDaysToRest;
    private Configuration configuration;

    public SantaCommunicator(int numberOfDaysToRest) {
        this.numberOfDaysToRest = numberOfDaysToRest;
    }

    public SantaCommunicator(Configuration configuration) {
        this(configuration.numberOfDaysToRest());
        this.configuration = configuration;
    }

    public String composeMessage(Reindeer reindeer, int numberOfDaysBeforeChristmas) {
        var daysBeforeReturn = daysBeforeReturn(reindeer.numbersOfDaysForComingBack(), configuration.numberOfDayBeforeChristmas());

        return "Dear " + reindeer.name() + ", please return from " + reindeer.location() +
                " in " + daysBeforeReturn + " day(s) to be ready and rest before Christmas.";
    }

    public boolean isOverdue(Reindeer reindeer, int numberOfDaysBeforeChristmas, Logger logger) {
        if (daysBeforeReturn(reindeer.numbersOfDaysForComingBack(), configuration.numberOfDayBeforeChristmas()) <= 0) {
            logger.log("Overdue for " + reindeer.name() + " located " + reindeer.location() + ".");
            return true;
        }
        return false;
    }

    private int daysBeforeReturn(int numbersOfDaysForComingBack, int numberOfDaysBeforeChristmas) {
        return configuration.numberOfDayBeforeChristmas() - numbersOfDaysForComingBack - configuration.numberOfDaysToRest();
    }
}

• We can now clean our code
  • Our IDE is giving us a lot of hints on that

• We use those hints to refactor the code

• We end up with this code

public class SantaCommunicator {
    private final Configuration configuration;

    public SantaCommunicator(Configuration configuration) {
        this.configuration = configuration;
    }

    public String composeMessage(Reindeer reindeer) {
        var daysBeforeReturn = daysBeforeReturn(reindeer.numbersOfDaysForComingBack());

        return "Dear " + reindeer.name() + ", please return from " + reindeer.location() +
                " in " + daysBeforeReturn + " day(s) to be ready and rest before Christmas.";
    }

    public boolean isOverdue(Reindeer reindeer, Logger logger) {
        if (daysBeforeReturn(reindeer.numbersOfDaysForComingBack()) <= 0) {
            logger.log("Overdue for " + reindeer.name() + " located " + reindeer.location() + ".");
            return true;
        }
        return false;
    }

    private int daysBeforeReturn(int numbersOfDaysForComingBack) {
        return configuration.numberOfDayBeforeChristmas() - numbersOfDaysForComingBack - configuration.numberOfDaysToRest();
    }
}

• We clean as well the tests and end up with

class SantaCommunicatorTest {
    private final int numberOfDaysToRest = 2;
    private final int numberOfDayBeforeChristmas = 24;
    private final TestLogger logger = new TestLogger();
    private SantaCommunicator communicator;

    @BeforeEach
    void setup() {
        this.communicator = new SantaCommunicator(new Configuration(numberOfDaysToRest, numberOfDayBeforeChristmas));
    }

    @Test
    void composeMessage() {
        assertThat(communicator.composeMessage(
                reindeer(5)
        )).isEqualTo("Dear Dasher, please return from North Pole in 17 day(s) to be ready and rest before Christmas.");
    }

    @Test
    void shouldDetectOverdueReindeer() {
        var overdue = communicator.isOverdue(
                reindeer(numberOfDayBeforeChristmas),
                logger);

        assertThat(overdue).isTrue();
        assertThat(logger.getLog())
                .isEqualTo("Overdue for Dasher located North Pole.");
    }

    @Test
    void shouldReturnFalseWhenNoOverdue() {
        assertThat(communicator.isOverdue(
                reindeer(numberOfDayBeforeChristmas - numberOfDaysToRest - 1),
                logger)
        ).isFalse();
    }

    private static Reindeer reindeer(int numbersOfDaysForComingBack) {
        return new Reindeer("Dasher", "North Pole", numbersOfDaysForComingBack);
    }
}

Reflect

• When was the last time you observed primitive types everywhere?
• `How could it be useful to use the refactoring practiced here?
• Which cohesive data structures could be introduced in your current project?