Question

Is there a language which has a feature that can prevent a class accessing any other class, unless an instance or reference is contained?

isolated class Example {

    public Integer i;

    public void doSomething()
    {
        i = 5; // This is ok because i belongs to this class

        /*
         * This is forbidden because this class can only
         * access anything  contained within, nothing outside
         */
        System.out.println("This does not work.");
    }
}

[edit]An example use case might be a plugin system. I could define a plugin object with references to certain objects that class can manipulate, but nothing else is permissible. It could potentially make security concerns much easier.[/edit]

Answer 1

I’m not aware of any class-based access modifiers with such intent, but I believe access modifiers to be misguided anyway.

Capability-based security or, more specifically, the object-capability model seems to be what you want.

http://en.wikipedia.org/wiki/Object-capability_model

The basic idea is that in order to do anything with an object, you need to hold a reference to it. Withhold the reference and no access is possible.

Global things (such as System.out.println) and a few other things are problematic features of a language, because anyone can access them without a reference.

Languages such as E, or tools like google caja (for Javascript) allow proper object-capability models. Here an example in JS:

function Example(someObj) {

    this.someObj = someObj;

    this.doStuff() = function() {
        this.someObj.foo(); //allowed, we have been given a reference to it
        alert("foobar"); //caja may deny/proxy access to global "alert"
    }
}