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    public class ImmutableIntArray
    {
        private int[] data;

        public ImmutableIntArray (int[] data)
        {
            this.data = data;
        }

        int getElementAt (int index)
        {
            return data[index];
        }
    }

    public class Main
    {
        static public void main (String[] args)
        {
            try
            {
                System.out.print ("This should work ...");
                ValidExampleCaller.sampleMethod();
                System.out.println (" and it did.");
            }
            catch (InvalidKeyException e1)
            {
                e1.printStackTrace();
            }

            try
            {
                System.out.println ("This should fail ...");
                InValidExampleCaller.sampleMethod1(); 
            }
            catch (InvalidKeyException e2)
            {
                e2.printStackTrace();
            }
        }
    }    

 /**
     *
     */
    class InvalidKeyException extends java.lang.Exception
    {
        public InvalidKeyException (String message)
        {
            super (message);
        }
    }

    class ExampleCallee
    {
        /**
         * This method prints 'Hi!' to standard out, but can only
         * be called by the ExampleCaller class.
         *
         * @param key 'key' is an object restricting access to this
         *            method.  If 'key' is not an instance of
         *            class ValidExampleCaller.Key, an InvalidKeyException
         *            is thrown.
         */
        public static void sampleMethod (Object key) throws InvalidKeyException
        {
            if (key.getClass().getName().equals ("ValidExampleCaller$Key") == false)
                throw new InvalidKeyException("Can't call sampleMethod() with key " + key);

            System.out.println ("Hi!");
        }
    }

    class ValidExampleCaller
    {
        static private final class Key
        {
        }

        static public void sampleMethod () throws InvalidKeyException
        {
            ExampleCallee.sampleMethod (new Key());    
        }
    }

    class InValidExampleCaller
    {
        static private final class Key
        {
        }

        static public void sampleMethod1 () throws InvalidKeyException
        {
            ExampleCallee.sampleMethod (new Key());    
        }
    }