SKB – Scala main


This article is part of the Scala knowledge bits Series.

Periodically, I will publish new exercises so you can slowly build up knowledge about Scala.

It is designed to be done in a very short amount of time and learn a little bit each day, just to create a routine.

This episode will teach you about Scala main.

Hope you are going to enjoy it! It is designed for anyone to learn Scala from scratch and slowly learn, one Bit at a time.

After this Bit, I would love to hear your feedback in the comments down below.

Feel free to join the Discord server as well if you would like some help and support from the rest of our community.

What are we learning today?

Today we are going to learn about Scala main !

A main is the method that is being called to start your program. It needs a specific structure – called prototype when talking about methods – to be recognized.

Until now we ran all the SKBs in Worksheet mode which hides the main from us.

Time to try on the exercise on your own and scroll down for more information when you are done or if you are stuck.


Here is an exercise to complete today.

If I did my job well, you should be able to guess by yourself the solution based on what you previously learned and based on the clues.

But if you get stuck, scroll down to get more information.

The goal of the exercise is to replace the ??? by a piece of code so that the exercise compiles and that’s how you win! Good luck!

You can fill the exercise right in here:

Or, if it does not load, go on to Scastie (Eb9UJewvRlOeORHTpjD5lQ).

More information about Scala main

In this exercise you will learn (or have learned, if you have already solved the puzzle) about Scala main.

If you have tried to remove or change the main function, you might have noticed an exception:

java.lang.RuntimeException: No main class detected.

This Exception will be thrown if a main method cannot be found or is not contained within an object. Try to change the code to trigger it.

The main prototype is always of the form:

def main(args: Array[String]): Unit = { }
The args: Array[String] is where you would be able to read and use the arguments given to the application when started.

That is pretty much it concerning main, so I thought it would be nice to combine some of the things we’ve seen in the past. This SKB is using Map, object, Option, lazy val, def all into the same project. Try to play with it and make it your own.

Feel free to go back to the exercise, modify the code to try out new things and get a better intuition for Scala main.


I hope you have learned something new or had fun during this Scala Knowledge Bit.

Please ask questions or post feedback in the comments below.

Feel free to try on the next Scala Knowledege Bit.

If you are curious about the previous Scala knowledge Bits, go check it out! 🙂

2 thoughts on “SKB – Scala main”

  1. Hello Leo,
    Thank you for such a great series about Scala.

    I remember the SKB about lazy modifier and what it is for, but I still can’t justify in my mind the lazy modifier usage for the fakeDatabase field. I agree that until this value is accessed, we won’t allocate memory for it, but is there any other benefits to use it in this particular example?

    Thank you!

    • Thank you !

      An example is usually for testing.

      In this case the database is hardcoded but it could be an actual DatabaseConnector that hold a connection to the database.

      Then a function, for instance def getUserTable, would use the DatabaseConnector.

      The problem occur when you want to test. You will probably want to override getUserTable but if the DatabaseConnector is a val instead of a lazy val it will be instantiated even tho you will never need it during your test.

      If you have more questions, feel to join the discord community and ask in the #skb channel 🙂


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