Java static methods accessing private variables

Question

I was under the impression that private non-static variables could only be accessed by methods called on the object that the variables reside in, but this is not the case. Could someone please explain the reasoning behind why the following compiles and runs?

public class Sandbox {
    private String _privateString = "unmodified";
    public static void setPrivateString(String str, Sandbox s) {
        s._privateString = str;
    }
    public String toString()
    {
        return _privateString;
    }

    public static void main(String[] args) {
        Sandbox s = new Sandbox();
        setPrivateString("modified", s);
        System.out.println(s);
    }
}

Output:

modified

EDIT: The same is true in C#.

Answer 1

Private member variables of class A can be accessed (i.e. read/written to) by any method of class A (static or non-static), so in your example, since the method changing the string is a method of the same class the member belongs to, it is granted access to the variable.

The reason is because a class is considered a self-contained body of logic (i.e. a specific implementation), so it makes sense that privacy is contained within a class; there is no reason to exclude static methods from that access right, since they are also part of the specific implementation the class provides.

Answer 2

You seem to be confusing visibility with scope. The instance variables are in the scope of an instance, so they cannot be accessed in a static method directly, but only with an instance reference qualifier: s._privateString in your case.

However, this does not mean that instance variables are not visible for a static method inside the same class, as private means visible inside the class (for any member with any scope).