Can Java Replicate the C 'Friend' Concept?

Can You Replicate the C 'Friend' Concept in Java?

In Java, it is generally not possible for a class in one package to access non-public members of a class in another package. However, there is a trick that can be used to simulate this behavior.

The Trick

The trick involves using a nested class with a private constructor. This nested class acts as a "signature security" for the method that you want to restrict access to. Here's an example:

// Package: capulet
public class Juliet {

    public static void cuddle(Romeo.Love love) {
        // Check that the love is real (null check)
        Objects.requireNonNull(love);
        
        // Call the cuddle method
        System.out.println("O Romeo, Romeo, wherefore art thou Romeo?");
    }
    
}

// Package: montague
public class Romeo {
    // Nested class with private constructor
    public static final class Love { private Love() {} }
    
    // Static reference to the love class
    private static final Love love = new Love();
    
    // Public method to cuddle Juliet using the nested class
    public static void cuddleJuliet() {
        Juliet.cuddle(love);
    }
}

In this example, the Romeo.Love class is nested within the Romeo class and has a private constructor. This means that no other class can construct an instance of Romeo.Love. The Juliet class has a public method called cuddle that takes an instance of Romeo.Love as a parameter.

Since only the Romeo class can create instances of Romeo.Love, only the Romeo class can call the cuddle method. This effectively restricts access to the cuddle method to the Romeo class, simulating the behavior of the C friend concept.

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