Object-private variables implementation

Question

I'm trying to understand implementation of object-private variables in Scala. Scala compiles this class

class Counter{
    private[this] var age = 0
}

into the following java byte code:

public class Counter implements scala.ScalaObject {
  private int age;
  public Counter();
}

But still, because JVM doesn't support object-private fields, we have good-old private field, which can be accessed from other instances of the class. So for me the difference between previous class and the following in terms of hiding private field is not clear.

class Counter2{
    private var age = 0
}

public class Counter2 implements scala.ScalaObject {
  private int age;
  private int age();
  private void age_$eq(int);
  public Counter2();
}

What is your question? The obvious difference is that it is a compile-time error for another instance to access the object-private field. But I assume you know that, so I am not sure what is being asked here. — Ben James
– Ben James, Commented May 9, 2013 at 9:47
My question is how it is implemented, if JVM doesn't support object-private fields — Konstantin Milyutin
– Konstantin Milyutin, Commented May 9, 2013 at 9:50

Jörg W Mittag · Accepted Answer · 2013-05-09 11:30:33Z

1

The JVM is irrelevant. The semantics of Scala are implemented by the Scala compiler, not the JVM. After all, the JVM isn't even the only platform Scala runs on, there are production-ready implementations of Scala on the CLI, and experimental ones on ECMAScript as well as a native one.

answered May 9, 2013 at 11:30

Jörg W Mittag

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3 Comments

Konstantin Milyutin Over a year ago

so Scala compiler forbids all access to object-private fields, except those accessed through this? is it correct?

James_pic Over a year ago

It's worth noting that even the Java compiler sometimes produces bytecode that allows access to private variables and methods. For example, if an inner class accesses a private variable in its outer class. The reason is the same as for Scala - the JVM's data model isn't a perfect fit with the data models in either Scala or the latest versions of Java, so the compiler cheats a little bit.

Jörg W Mittag Over a year ago

And of course almost all JVMs eventually compile to native code, which doesn't know anything about objects, classes, interfaces, public, private, protected, methods, etc.

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