Type-safe Builder Library for Scala and Java

问题:

Below is a type-safe, fluid, builder pattern in Scala as described at http://www.tikalk.com/java/blog/type-safe-builder-scala-using-type-constraints. It's similar to Builder Library for Scala and Java, but deals specifically with compile-time builder checks. How can this called from Java? Can it be done with a clean API for Scala AND Java given the "scala.Predef$$eq$colon$eq" parameters?

sealed trait TBoolean
sealed trait TTrue extends TBoolean
sealed trait TFalse extends TBoolean

class Builder[HasProperty <: TBoolean] private(i: Int) {
  protected def this() = this(-1)
  def withProperty(i: Int)(implicit ev: HasProperty =:= TFalse) = new Builder[TTrue](i)
  def build(implicit ev: HasProperty =:= TTrue) = println(i)
}

//javap output
    public class Builder extends java.lang.Object implements scala.ScalaObject{
    public Builder withProperty(int, scala.Predef$$eq$colon$eq); //How is this called from Java?
    public void build(scala.Predef$$eq$colon$eq);
    public Builder();
}
object Builder {
  def apply() = new Builder[TFalse]
}

回答1:

You should be able to use this API from Java, with some extra noise compared to the Scala version. A few convenience fields will quiet things a bit:

object Builder {
   def apply() = new Builder[TFalse]
   val unassigned = =:=.tpEquals[TFalse]
   val assigned = =:=.tpEquals[TTrue] 
}

The Java client code should end up looking like

Builder$.MODULE$.apply()
   .withProperty(10, Builder$.MODULE$.unassigned())
   .build(Builder$.MODULE$.assigned());

The build method has to check that every property is assigned, so it gets pretty noisy when you generalize to multiple properties:

Builder$.MODULE$.apply()
   .withProp1(10, Builder$.MODULE$.unassigned())
   .withProp2(20, Builder$.MODULE$.unassigned())
   .withProp3(30, Builder$.MODULE$.unassigned())
   // ...
   .build(Builder$.MODULE$.assigned(), 
          Builder$.MODULE$.assigned(), 
          Builder$.MODULE$.assigned(), 
          //...
         );

With some static delegates in a helper class (and some static imports), you should be able to get this down to something like:

createBuilder()
   .withProp1(10, unassigned())
   .withProp2(20, unassigned())
   .build(assigned(), assigned());

回答2:

OK, thanks to Aaron and ittayd...here is a builder with a fluid API for Scala and Java:

import annotation.bridge

sealed trait TBoolean
sealed trait TTrue extends TBoolean
sealed trait TFalse extends TBoolean

class Builder[HasProperty <: TBoolean] private(i: Int) {
  protected def this() = this(-1)

  def withProperty(i: Int)(implicit ev: HasProperty =:= TFalse) = new Builder[TTrue](i)
  def build(implicit ev: HasProperty =:= TTrue):Int = i
  @bridge def withProperty(i: Int) = new Builder[TTrue](i)
  @bridge def build = build(null)
}

object Builder {
  def apply() = new Builder[TFalse]
  val unassigned = =:=.tpEquals[TFalse]
  val assigned = =:=.tpEquals[TTrue]
}

Scala Usage (Type Safe):

val v = Builder().withProperty(2).build

Java Usage I (type safe, but ugly):

int i = Builder$.MODULE$.apply()
        .withProperty(5, Builder$.MODULE$.unassigned())
         .build(Builder$.MODULE$.assigned()); 

Java Usage II (not type safe, but much cleaner, using @Bridge calls):

static Builder createBuilder() {
  return Builder$.MODULE$.apply();
}
...
int j = createBuilder().withProperty(7).build();