scala-code-compilation.md

Topic	Scala code compilation
Git sample	ListOperationsTest.scala

Scala class compilation

• Program below creates an object named ListOperationsTest

  object ListOperationsTest {
  
  	  /* Average method uses left folding technique */
  	def avg(values: List[Double]) = {
  		val sum = values.foldLeft(0.0) { _ + _ }
  		sum / values.size.toDouble
  	}
  }

• Compilation of above object results into following 3 classes:

  • ListOperationsTest$$anonfun$1.class : The anonymous Closure ({ _ + _ }) got compiled into it. Every Scala Closure results into an anonymous class, similar to a Java Lambda
  • ListOperationsTest.class : Actual call is forwarded to ListOperationsTest$

    public final class ListOperationsTest
    {
      public static double avg(List<Object> paramList)
      {
        return ListOperationsTest..MODULE$.avg(paramList);
      }
    }

  • ListOperationsTest$.class

    public final class ListOperationsTest$ {
      
      public static final  MODULE$;
    
      static { new ();
      }
    
      public double avg(List<Object> values) { 
      double sum = scala.runtime.BoxesRunTime.unboxToDouble(values.foldLeft(scala.runtime.BoxesRunTime.boxToDouble(0.0D), new ListOperationsTest..anonfun.1()));
      return sum / values.size(); 
      }
    
      private ListOperationsTest$() { MODULE$ = this; }
    }

• Porgram above are Java programs, decompiled from scala byte code

• Scala uses Singleton Objects mechanism, to ensure that they’re true objects but look similar to static method invocations to Java

Scala switch statement compilation

• Scala uses the tableswitch optimization. The tableswitch instruction is made up of mappings of integer values to bytecode instruction labels. Following Scala class is compiled into tableswitch statement

  def unannotated(x: Int) = x match {
  	case 1 => "One"
  	case 2 => "Two!"
  	case z => z + "?"
  }

  /* Equivalent bytecode of above Scala method */
  public java.lang.String unannotated(int);
  	Code:
  	0: iload_1
  	
  	1: tableswitch{ 
  		/* Mappings of integer values to bytecode instruction labels (or line numbers) */
  		1: 51;
  		2: 46;
  		default: 24
  	}
  ...