How Can Java Mimic C 's Friend Class Functionality?

Replicating C 'friend' Concept in Java

In C , the 'friend' concept allows classes in different packages to access non-public members of other classes. Java lacks a direct equivalent, but it provides a clever workaround using nested classes.

Implementation

Consider two classes, Romeo and Juliet, from different packages. Romeo wishes to access Juliet's non-public methods without subclassing her. Here's how to achieve it using nested classes:

// Juliet's package
public class Juliet {
    private void cuddle() {
        System.out.println("O Romeo, Romeo, wherefore art thou Romeo?");
    }
    
    // Signature security for cuddle
    public static class CuddleAllowed { private CuddleAllowed() {} }
    private static final CuddleAllowed cuddleAllowed = new CuddleAllowed();
    
    public void cuddle(Juliet.CuddleAllowed cuddle) {
        cuddle(); // Juliet can cuddle herself
        if (cuddle == cuddleAllowed) {
            System.out.println("Only Romeo can cuddle Juliet.");
        }
    }
}

// Romeo's package
public class Romeo {
    public static void cuddleJuliet() {
        Juliet juliet = new Juliet();
        juliet.cuddle(Juliet.cuddleAllowed); // Romeo can cuddle Juliet
    }
}

In this example:

• Juliet.cuddleAllowed is a nested class with a private constructor. Only Juliet can instantiate it.
• Juliet.cuddle() is private but can be accessed internally or from Juliet.CuddleAllowed instances.
• Romeo uses the cuddleAllowed instance to access cuddle(). Juliet will cuddle only if the instance is provided by Romeo.

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