private[this] vs private

Question

In Scala I see such feature as object-private variable. From my not very rich Java background I learnt to close everything (make it private) and open (provide accessors) if

Answer 1

This was tested using scala 2.11.5. Consider the code below

class C(private val x: Int) {
  override def equals(obj: Any) = obj match {
    case other: C => x == other.x
    case _ => false
  }
}

println(new C(5) == new C(5)) //true
println(new C(5) == new C(4)) //false

it will compile and work as this java (1.8) code

class C {
    private int x;

    public C(int x) {
        this.x = x;
    }

    public boolean equals(Object obj) {
        if (obj instanceof C) {
            return ((C) obj).x == x;
        }
        else {
            return false;
        }
    }
}

System.out.println(new C(5).equals(new C(5))); //true
System.out.println(new C(5).equals(new C(4))); //false

however if you use '[this]' modifier the code below won't compile

class C(private[this] val x: Int) {
  override def equals(obj: Any) = obj match {
    case other: C => this.x == other.x //problem is here
    case _ => false
  }
}

This is because in the first case 'x' is accessible on class level, whereas in the second case is it more strict instance level. It means that 'x' can be accessed only from the instance to which it belongs. So 'this.x' is fine but 'other.x' is not.

You can refer to section 13.5 of "Programming in Scala: A Comprehensive Step-By-Step Guide" book for more details about access modifiers.