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    <p><strong>Here is a small trick that I use in JAVA to replicate C++ friend mechanism.</strong></p> <p>Lets say I have a class <code>Romeo</code> and another class <code>Juliet</code>. They are in different packages (family) for hatred reasons.</p> <p><code>Romeo</code> wants to <code>cuddle</code> <code>Juliet</code> and <code>Juliet</code> wants to only let <code>Romeo</code> <code>cuddle</code> her.</p> <p>In C++, <code>Juliet</code> would declare <code>Romeo</code> as a (lover) <code>friend</code> but there are no such things in java.</p> <p>Here are the classes and the trick :</p> <p>Ladies first :</p> <pre class="lang-java prettyprint-override"><code>package capulet; import montague.Romeo; public class Juliet { public static void cuddle(Romeo.Love l) { l.hashCode(); System.out.println("O Romeo, Romeo, wherefore art thou Romeo?"); } } </code></pre> <p>So the method <code>Juliet.cuddle</code> is <code>public</code> but you need a <code>Romeo.Love</code> to call it. It uses this <code>Romeo.Love</code> as a "signature security" to ensure that only <code>Romeo</code> can call this method and simply calls <code>hashCode</code> on it so the runtime will throw a <code>NullPointerException</code> if it is <code>null</code>.</p> <p>Now boys :</p> <pre class="lang-java prettyprint-override"><code>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); } } </code></pre> <p>The class <code>Romeo.Love</code> is public, but its constructor is <code>private</code>. Therefore anyone can see it, but only <code>Romeo</code> can construct it. I use a static reference so the <code>Romeo.Love</code> that is never used is only constructed once and does not impact optimization.</p> <p>Therefore, <code>Romeo</code> can <code>cuddle</code> <code>Juliet</code> and only he can because only he can construct and access a <code>Romeo.Love</code> instance, which is required by <code>Juliet</code> to <code>cuddle</code> her (or else she'll slap you with a <code>NullPointerException</code>).</p>
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