# SKB – Scala multiple inheritance

## Introduction

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 multiple inheritance.

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 multiple inheritance !

We talked about inheritance in the past.

Let’s see how we can combine several `trait` together and what are the limitations.

For this episode, finish the exercise once and then go back and follow the extra instructions in the comments if you have time to learn more.

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

## Exercise

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.

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 (QpCO0GjsTrKBo4wGdIpEJA).

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

The keyword of the day is `with`.

This article is for scala 2.x, things are changing in scala 3.x but the concepts are still the same. At the time of this article, Scastie, which I use to render the exercise do not support Scala 3 yet so we are going to revisit those concept and the new syntaxes in the future.

We saw `extends` already, when we want to add one more `trait` to the mix, we use `with` for the one after the first one.

```[case] class MyClass(args....) extends TRAIT1 with TRAIT2 with TRAIT3 .... {

}
```

If you have followed the extra instructions in the comments after you were done with the exercise, you noticed that `trait` and `abstract class` behave differently.

You have to put the `abstract class` in first position and you can only have one present. It can seems weird but in practice, it is rarely an issue since you can convert the `abstract class` to a `trait` very easily. You would just move the input arguments to the body of the `class` and make them methods with `def`.

You might have also noticed that you can mix two or more `trait` that have the same method(s) defined as long as they are identical. But it stops working if they have methods with the same name but different types.

And the last bits to keep in mind is when using defined type, you cannot use that has a shortcut to create larger `trait`. But you can use it in the position of type. Let me explain:

```type FOO = A with B

// this do no work:
trait BAR extends FOO

// this works:
trait BAR extends A with B

// this works:
val f: FOO = ???
```

If you want to create bundle of `trait`, you can simply make a new `trait`

```trait FOO extends A with B

trait BAR extends FOO

val f: FOO = new BAR {}
```

And that is it for mixing several `trait` and handle multiple inheritance. As always, if you have questions or feedbacks, jump on the Discord server ! See you there.

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

## Conclusion

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