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The Obligatory Hello World
##########################

Since every programming paradigm needs to solve the tough problem of printing a
well-known greeting to the console we’ll introduce you to the actor-based
version.

.. includecode:: ../scala/code/docs/actor/IntroDocSpec.scala#hello-world

The ``HelloWorld`` actor is the application’s “main” class; when it terminates
the application will shut down—more on that later. The main business logic
happens in the :meth:`preStart` method, where a ``Greeter`` actor is created
and instructed to issue that greeting we crave for. When the greeter is done it
will tell us so by sending back a message, and when that message has been
received it will be passed into the behavior described by the :meth:`receive`
method where we can conclude the demonstration by stopping the ``HelloWorld``
actor. You will be very curious to see how the ``Greeter`` actor performs the
actual task:

.. includecode:: ../scala/code/docs/actor/IntroDocSpec.scala#greeter

This is extremely simple now: after its creation this actor will not do
anything until someone sends it a message, and if that happens to be an
invitation to greet the world then the ``Greeter`` complies and informs the
requester that the deed has been done.

As a Scala developer you will probably want to tell us that there is no
``main(Array[String])`` method anywhere in these classes, so how do we run this
program? The answer is that the appropriate :meth:`main` method is implemented
in the generic launcher class :class:`akka.Main` which expects only one command
line argument: the class name of the application’s main actor. This main method
will then create the infrastructure needed for running the actors, start the
given main actor and arrange for the whole application to shut down once the
main actor terminates. Thus you will be able to run the above code with a
command similar to the following::

  java -classpath <all those JARs> akka.Main com.example.HelloWorld

This conveniently assumes placement of the above class definitions in package
``com.example`` and it further assumes that you have the required JAR files for
``scala-library`` and ``akka-actor`` available. The easiest would be to manage
these dependencies with a build tool, see :ref:`build-tool`.