C++-like friend class mechanism in Java

Question

Do you know how can I make an object changeable only inside a special class? In this example I want the object PrivateObject to be only changable (incrementable) inside the

Answer 1

Java does not have any language feature equivalent of C++'s friend keyword. However , there are number of application-level alternatives :

1. Declare the increment() method of the PrivateObject class as package- private and define Box in the same package.

OR

2.Let the calling code pass a token into increment() - inside this method check if this token is indeed coming from Box class and allow or disallow.

The idea is to keep the coupling between PrivateObject and Box class manageable.