private[this] vs private in scala

gistfile1.txt

// Foo.scala
class Foo {
    private var i = 0 
    private[this] var j = 0 
    def getJ = j 
    def iThing = i * i 
    def jThing = j * j 
}

object Foo {
    def main(args: Array[String]) {
        val f = new Foo 
        f.i 
        f.getJ
        f.iThing
        f.jThing
    }   
}

// scalac Foo.scala && javap -c Foo$class
Compiled from "Foo.scala"
public class Foo {
  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #16                 // Field Foo$.MODULE$:LFoo$;
       3: aload_0       
       4: invokevirtual #18                 // Method Foo$.main:([Ljava/lang/String;)V
       7: return        

  public int Foo$$i();
    Code:
       0: aload_0
       1: getfield      #24                 // Field Foo$$i:I
       4: ireturn

  public int getJ();
    Code:
       0: aload_0
       1: getfield      #32                 // Field j:I
       4: ireturn

  public int iThing();
    Code:
       0: aload_0
       1: invokevirtual #35                 // Method Foo$$i:()I
       4: aload_0
       5: invokevirtual #35                 // Method Foo$$i:()I
       8: imul
       9: ireturn

  public int jThing();
    Code:
       0: aload_0
       1: getfield      #32                 // Field j:I
       4: aload_0
       5: getfield      #32                 // Field j:I
       8: imul
       9: ireturn

  public Foo();
    Code:
       0: aload_0
       1: invokespecial #40                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: iconst_0
       6: putfield      #24                 // Field Foo$$i:I
       9: aload_0
      10: iconst_0
      11: putfield      #32                 // Field j:I
      14: return
}

note the difference between iThing and jThing, one uses a field access and the other a virtual method. if you want to be able to expose your fields to the public, but want fast access with getfield, then use private[this] and write your own getter.

Hi,twolfe18, what do you use to view the byte code? Thanks.

Here is my decompiled code. I just removed some methods:

def getJ = j 
def iThing = i * i 
def jThing = j * j 

Plus

f.i 
f.getJ
f.iThing
f.jThing