Can I limit the methods another class can call in Java?

Question

Let's assume I have classes A, B, and C where class C has readable and writable properties:

public class C {
    private int i = 0;

    // Writable.
    public void increment() { i++; }

    // Readable.
    public int getScore() { return i; }
}

Is it possible to only let A use the increment() method and only let B use the getScore() method?

Answer 1

No, it is not possible to assign per-class access.

Consider separating your class into separate interfaces so that each class only gets an object with the interface it needs. For example:

interface Incrementable { public void increment(); }
interface HasScore { public int getScore(); }
class C implements Incrementable, HasScore { /* ... */ }

class A {
  public A(Incrementable incr) { /* ... */ }
}

class B {
  public B(HasScore hs) { /* ... */ }
}

Of course, there are security implications but this should get you thinking in the right direction.

Answer 2

Yes it is, but you have to go through some gyrations.

public interface Incrementable {
    public void increment();
}

public interface Readable {
    public int getScore();
}

public class C implements Incrementable, Readable
{
    ...
}

Now when you define the method in A that receives a reference to a B instance, define that method to take an Incrementable instead. For B, define it to take a Readable.