Is there a way to simulate the C++ 'friend' concept in Java?

Question

I would like to be able to write a Java class in one package which can access non-public methods of a class in another package without having to make it a subclass of the other class. Is this possible?

Answer 1

Here is a small trick that I use in JAVA to replicate C++ friend mechanism.

Lets say I have a class Romeo and another class Juliet. They are in different packages (family) for hatred reasons.

Romeo wants to cuddle Juliet and Juliet wants to only let Romeo cuddle her.

In C++, Juliet would declare Romeo as a (lover) friend but there are no such things in java.

Here are the classes and the trick :

Ladies first :

package capulet;

import montague.Romeo;

public class Juliet {

    public static void cuddle(Romeo.Love love) {
        Objects.requireNonNull(love);
        System.out.println("O Romeo, Romeo, wherefore art thou Romeo?");
    }

}

So the method Juliet.cuddle is public but you need a Romeo.Love to call it. It uses this Romeo.Love as a "signature security" to ensure that only Romeo can call this method and checks that the love is real so that the runtime will throw a NullPointerException if it is null.

Now boys :

package montague;

import capulet.Juliet;

public class Romeo {
    public static final class Love { private Love() {} }
    private static final Love love = new Love();

    public static void cuddleJuliet() {
        Juliet.cuddle(love);
    }
}

The class Romeo.Love is public, but its constructor is private. Therefore anyone can see it, but only Romeo can construct it. I use a static reference so the Romeo.Love that is never used is only constructed once and does not impact optimization.

Therefore, Romeo can cuddle Juliet and only he can because only he can construct and access a Romeo.Love instance, which is required by Juliet to cuddle her (or else she'll slap you with a NullPointerException).