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
     }
  ...