What is the Scala equivalent to a Java builder pattern?

You have three main alternatives here.

1. Use the same pattern as in Java, classes and all.

2. Use named and default arguments and a copy method. Case classes already provide this for you, but here's an example that is not a case class, just so you can understand it better.

   object Size {
       sealed abstract class Type
       object Large extends Type
   }
   
   object Base {
       sealed abstract class Type
       object Cheesy extends Type
   }
   
   object Ingredient {
       sealed abstract class Type
       object Ham extends Type
   }
   
   class Pizza(size: Size.Type, 
               base: Base.Type, 
               ingredients: List[Ingredient.Type])
   
   class PizzaBuilder(size: Size.Type, 
                      base: Base.Type = null, 
                      ingredients: List[Ingredient.Type] = Nil) {
   
       // A generic copy method
       def copy(size: Size.Type = this.size,
                base: Base.Type = this.base,
                ingredients: List[Ingredient.Type] = this.ingredients) = 
           new PizzaBuilder(size, base, ingredients)
   
   
       // An onTopOf method based on copy
       def onTopOf(base: Base.Type) = copy(base = base)
   
   
       // A with method based on copy, with `` because with is a keyword in Scala
       def `with`(ingredient: Ingredient.Type) = copy(ingredients = ingredient :: ingredients)
   
   
       // A build method to create the Pizza
       def build() = {
           if (size == null || base == null || ingredients == Nil) error("Missing stuff")
           else new Pizza(size, base, ingredients)
       }
   }
   
   // Possible ways of using it:
   new PizzaBuilder(Size.Large).onTopOf(Base.Cheesy).`with`(Ingredient.Ham).build();
   // or
   new PizzaBuilder(Size.Large).copy(base = Base.Cheesy).copy(ingredients = List(Ingredient.Ham)).build()
   // or
   new PizzaBuilder(size = Size.Large, 
                    base = Base.Cheesy, 
                    ingredients = Ingredient.Ham :: Nil).build()
   // or even forgo the Builder altogether and just 
   // use named and default parameters on Pizza itself
   

3. Use a type safe builder pattern. The best introduction I know of is this blog, which also contains references to many other articles on the subject.

   Basically, a type safe builder pattern guarantees at compile time that all required components are provided. One can even guarantee mutual exclusion of options or arity. The cost is the complexity of the builder code, but...

Another alternative to the Builder pattern in Scala 2.8 is to use immutable case classes with default arguments and named parameters. Its a little different but the effect is smart defaults, all values specified and things only specified once with syntax checking...

The following uses Strings for the values for brevity/speed...

scala> case class Pizza(ingredients: Traversable[String], base: String = "Normal", topping: String = "Mozzarella")
defined class Pizza

scala> val p1 = Pizza(Seq("Ham", "Mushroom"))                                                                     
p1: Pizza = Pizza(List(Ham, Mushroom),Normal,Mozzarella)

scala> val p2 = Pizza(Seq("Mushroom"), topping = "Edam")                               
p2: Pizza = Pizza(List(Mushroom),Normal,Edam)

scala> val p3 = Pizza(Seq("Ham", "Pineapple"), topping = "Edam", base = "Small")       
p3: Pizza = Pizza(List(Ham, Pineapple),Small,Edam)

You can then also use existing immutable instances as kinda builders too...

scala> val lp2 = p3.copy(base = "Large")
lp2: Pizza = Pizza(List(Ham, Pineapple),Large,Edam)

Case classes solve the problem as shown in previous answers, but the resulting api is difficult to use from java when You have scala collections in your objects. To provide a fluent api to java users try this:

case class SEEConfiguration(parameters : Set[Parameter],
                               plugins : Set[PlugIn])

case class Parameter(name: String, value:String)
case class PlugIn(id: String)

trait SEEConfigurationGrammar {

  def withParameter(name: String, value:String) : SEEConfigurationGrammar

  def withParameter(toAdd : Parameter) : SEEConfigurationGrammar

  def withPlugin(toAdd : PlugIn) : SEEConfigurationGrammar

  def build : SEEConfiguration

}

object SEEConfigurationBuilder {
  def empty : SEEConfigurationGrammar = SEEConfigurationBuilder(Set.empty,Set.empty)
}


case class SEEConfigurationBuilder(
                               parameters : Set[Parameter],
                               plugins : Set[PlugIn]
                               ) extends SEEConfigurationGrammar {
  val config : SEEConfiguration = SEEConfiguration(parameters,plugins)

  def withParameter(name: String, value:String) = withParameter(Parameter(name,value))

  def withParameter(toAdd : Parameter) = new SEEConfigurationBuilder(parameters + toAdd, plugins)

  def withPlugin(toAdd : PlugIn) = new SEEConfigurationBuilder(parameters , plugins + toAdd)

  def build = config

}

Then in java code the api is really easy to use

SEEConfigurationGrammar builder = SEEConfigurationBuilder.empty();
SEEConfiguration configuration = builder
    .withParameter(new Parameter("name","value"))
    .withParameter("directGivenName","Value")
    .withPlugin(new PlugIn("pluginid"))
    .build();