Thursday, August 20, 2009

Introducing a Type

An important building block when creating a high quality domain model is to create small type objects. In this article we will create a Length type for the diameter of the Planet of the helloworld application.



Alternative video format (mpg)

Length is a typical Quantity with a value and unit, e.g. meter, kilometer.

In the design model it looks like this:
BasicType Length {
BigDecimal value min="0"
-@LengthUnit unit
}

enum LengthUnit {
cm, m, km
}

Entity Planet {
gap
scaffold
String name key
Long population min="0"
-@Length diameter nullable
-Set<@Moon> moons opposite planet

Repository PlanetRepository {
findByKey;
}
}


We also need to convert between different units. The behaviour expressed as a JUnit test:
public class LengthTest {

@Test
public void shouldConvertFromMeterToKilometer() {
Length length = new Length(new BigDecimal("31000"), m);
Length lengthInKilometer = length.to(km);
assertEquals(new Length(new BigDecimal("31"), km),
lengthInKilometer);
}

@Test
public void shouldConvertFromKilometerToMeter() {
Length length = new Length(new BigDecimal("44"), km);
Length lengthInMeter = length.to(m);
assertEquals(new Length(new BigDecimal("44000"), m),
lengthInMeter);
}

@Test
public void shouldNotConvertSameUnit() {
Length length = new Length(new BigDecimal("17"), km);
Length length2 = length.to(km);
assertSame(length, length2);
}
}

BasicType objects may contain business logic in the same way as other domain objects. The following screencast illustrates how to implement the conversion.



Alternative video format (mpg)

BasicType is a stored in the same table as the Domain Object referencing it. It corresponds to JPA @Embeddable.

There are a lot of cases when it is a good idea to introduce types.
  • Identifers, natural business keys. It is more readable to pass around an identifier type instead of a plain String or Integer
  • Money
  • Range
  • Quantity
I can recommend reading When to Make a Type, Martin Fowler.

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