SKB – Scala companion objects


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 companion objects.

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 companion objects !

Companion objects are a specific type of object.

Just like the object we saw in the previous SKB, companion object does not need to be instantiated.

The source of the main difference between object and companion object comes from the name that they share with a class.

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 (cwwIb13GTha0xT9adIjMqw).

More information about Scala companion objects

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

Did you notice that the class named Animal is able to call a method named convertLegNumberToName that is defined as private?

Other thing to notice is the import. In the previous SKB related to object, we learned how to call the member of an object using its name. When you use the import, you do not need to use the name anymore. Inside the context, where the import is used, with the Animal._, the name is not needed to be called.

To summarize, a companion object is exactly like an object, except that the class that share the same name can access the private members.

It is commonly used when you need to have constant or helper methods related to this class.

Leaving those members inside the class can create memory issues. Let’s say you have a large constant – maybe an array or some other complex object. If it is stored inside the class, then each instantiation will make a copy of this constant in the memory. If instead the constant is in the companion object, then the program will only have to create the constant once.

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


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! 🙂

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