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Are duplicate arguments to a Scala constructor?

I just stumbled over some situation that seemed strange to me. Perhaps I really missed the obvious here - in any case, please help me.

Consider the following Scala repl script:

scala> class X(val s: String) { def run=println("(X): "+s) }
defined class X

scala> class Y(s: String) extends X("MY "+s) { override def run=println("(Y): "+s) }
defined class Y

scala> new Y("fish").run
(Y): fish

In the script, I define class X with the class attribute "val s". Then I define the class Y, which should take one argument of the constructor and pass it to the X that it makes. To show the difference, I modify "s" before passing it to X ("MY" + s).

Finally, I create a new Y and call "run". This displays the "fish" on the console, so obviously the attribute "s" of class "X" was obscured by the new attribute "s" that I created in "Y".

Scala 2.8 2.9.1 .

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+5
2

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, :

class X(val s: String) { def run=println("(X): "+s) }
class Y(s: String) extends X("MY "+s) { override def run=println("(Y): "+s) }
class Z(s0: String) extends X("MY "+s0) { override def run=println("(Z): "+s) }

- ( ):

// Note putfield to store s
public X(java.lang.String);
  Code:
   0:   aload_0
   1:   aload_1
   2:   putfield    #11; //Field s:Ljava/lang/String;
   5:   aload_0
   6:   invokespecial   #43; //Method java/lang/Object."<init>":()V
   9:   return

// Note putfield to store new s (then eventually calls X constructor)
public Y(java.lang.String);
  Code:
   0:   aload_0
   1:   aload_1
   2:   putfield    #29; //Field s:Ljava/lang/String;
   5:   aload_0
   6:   new #16; //class scala/collection/mutable/StringBuilder
   9:   dup
   10:  invokespecial   #19; //Method scala/collection/mutable/StringBuilder."<init>":()V
   13:  ldc #40; //String MY 
   15:  invokevirtual   #25; //Method scala/collection/mutable/StringBuilder.append:(Ljava/lang/Object;)Lscala/collection/mutable/StringBuilder;
   18:  aload_1
   19:  invokevirtual   #25; //Method scala/collection/mutable/StringBuilder.append:(Ljava/lang/Object;)Lscala/collection/mutable/StringBuilder;
   22:  invokevirtual   #33; //Method scala/collection/mutable/StringBuilder.toString:()Ljava/lang/String;
   25:  invokespecial   #44; //Method X."<init>":(Ljava/lang/String;)V
   28:  return

// Note - no putfield!
public Z(java.lang.String);
  Code:
   0:   aload_0
   1:   new #14; //class scala/collection/mutable/StringBuilder
   4:   dup
   5:   invokespecial   #17; //Method scala/collection/mutable/StringBuilder."<init>":()V
   8:   ldc #39; //String MY 
   10:  invokevirtual   #23; //Method scala/collection/mutable/StringBuilder.append:(Ljava/lang/Object;)Lscala/collection/mutable/StringBuilder;
   13:  aload_1
   14:  invokevirtual   #23; //Method scala/collection/mutable/StringBuilder.append:(Ljava/lang/Object;)Lscala/collection/mutable/StringBuilder;
   17:  invokevirtual   #32; //Method scala/collection/mutable/StringBuilder.toString:()Ljava/lang/String;
   20:  invokespecial   #43; //Method X."<init>":(Ljava/lang/String;)V
   23:  return
+8

, s . , (.. ) - . , Y.run , s Y.s.

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:

  • var = > , getter setter
  • val = > getter
  • none = > (.. , ), /
+1

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