A demonstration of how to make composition a first (or 1.5th) class citizen in Scala OO code

scala_composition.scala

// A demonstration of how to make composition a first (or 1.5th) class
// citizen in Scala OO code, as it is better to favor composition
// over inheritance.
//
// The basic idea is to create a sister trait named "MyTraitComposition" to your
// base trait. The composition trait specifies an abstract member "delegate"
// and delegates the full MyTrait interface to that delegate. Later on,
// concrete implementations can implement MyTrait with MyTraitComposition
// to easily extend the behavior of another existing concrete implementation
// via composition.
//
// This demonstration uses the example of a trait called Blobstore. Ignore
// the unsavory use of null.
//


/**
 * A place to store binary data. This is the base trait of which we
 * will be making multiple implementations 
 */
trait Blobstore {
  def put(keyValue: (String, AnyRef))

  def get(key: String): AnyRef

  def url(key: String): String
}

/**
 * Sister trait to our base trait -- this allows different implementations
 * to easily DELEGATE rather than INHERIT behavior from each other without
 * wasted syntax.
 */
trait BlobstoreComposition extends Blobstore {  
  protected def blobstoreDelegate: Blobstore

  override def put(keyValue: (String, AnyRef)) = {
    blobstoreDelegate.put(keyValue)
  }

  override def get(key: String): AnyRef = {
    blobstoreDelegate.get(key)
  }

  override def url(key: String): String = {
    blobstoreDelegate.url(key)
  }

}


/** 
 * First implementation: Let's go straight to S3 for everything 
 */
class S3Blobstore extends Blobstore {
  def put(keyValue: (String, AnyRef)) {
    println("PUT " + keyValue + "INTO S3 ")
  }

  def get(key: String): AnyRef = {
    println("GOT " + key + "FROM S3")
    null
  }

  def url(key: String): String = {
    "s3.amazonaws.com/mybucket/" + key
  }
}


/**
 * Second implementation: delegate to any other blobstore implementation, but
 * cache the results in a DB. Notice we only override what we have to override
 * (we choose not to override urlOption)
 */
class DBIndexedBlobstore (protected val blobstoreDelegate: Blobstore)
  extends Blobstore 
  with BlobstoreComposition 
{
  // Notably, we are overriding the trivial BlobstoreComposition implementation 
  // of get, not the delegate's.   
  override def put(keyValue: (String, AnyRef)) {
    // Cache the results here and then delegate.
    dbPut(keyValue)
    blobstoreDelegate.put(keyValue)
  }

  override def get(key: String): AnyRef = {    
    val fromDb = dbGet(key)
    
    if (fromDb == null) {
      blobstoreDelegate.get(key)
    } else {
      null
    }
  }

  //
  // Private members
  //
  private def dbPut(keyValue:(String, AnyRef)) = {
    null
  }

  private def dbGet(key: String): AnyRef = {
    null
  }
}


// Scala harness
object DecoratorTest {
  def main(args: Array[String]) {
    println("")
    println("It ran jist fine")
    println("")
  }
}