How Can I Simulate C 's `friend` Functionality in Java?

Simulating the C 'friend' Concept in Java

To grant direct access to private methods between classes from different packages in Java, consider the following technique:

Step 1: Create a "Security Signature" Class

Within the package of the class that desires access, define a public class that acts as a "security signature." This class should only be accessible within its package.

Step 2: Restrict Constructor

Make the constructor of the security signature class private. This ensures that only the class within its package can instantiate it.

Step 3: Create a Static Reference

Declare a static variable of type security signature class. This allows for easy access to the signature object.

Step 4: Define the Target Method

In the class that wishes to allow limited access, define a method that requires the security signature class as an argument. This method should have appropriate access permissions, such as public or protected.

Example:

Consider a scenario where class Romeo (in package Montague) wants to access non-public methods of class Juliet (in package Capulet).

Juliet.java:

package capulet;

public class Juliet {

    public void cuddle(Romeo.Love love) {
        if (love == null) {
            throw new NullPointerException();
        }
        System.out.println("O Romeo, Romeo, wherefore art thou Romeo?");
    }
}

Romeo.java:

package montague;

public class Romeo {

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

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

In this example, the class Romeo.Love acts as the security signature. Only Romeo can construct it due to its private constructor. The method cuddle in Juliet requires an instance of Romeo.Love to execute, which Romeo can provide. This prevents unauthorized access from other classes outside the Montague package.

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