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.



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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.



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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.