inherit

What def, val, and var fields in Scala traits look like after they’re compiled (including the classes that extend them)

Table of Contents1 - def field in trait2 - val field in trait (abstract)3 - val field in trait (concrete)4 - var field in trait (abstract)5 - var field in trait (concrete)6 - An abstract class in the middle7 - A trait in the middle8 - Summary

I generally have a pretty good feel for how Scala traits work, and how they can be used for different needs. As one example, a few years ago I learned that it’s best to define abstract fields in traits using def. But there are still a few things I wonder about.

Today I had a few free moments and I decided to look at what happens under the covers when you use def, val, and var fields in traits, and then mix-in or extend those traits with classes. So I created some examples, compiled them with scalac -Xprint:all, and then decompiled them with JAD to see what everything looks like under the covers.

I was initially going to write a summary here, but if you want to know how things work under the hood, I think it helps to work through the examples, so for today I’ll leave that as an exercise for the reader.

How to dynamically add a Scala trait to an object instance

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is a very short recipe, Recipe 8.8, “How to dynamically add a Scala trait to an object instance.”

Problem

Rather than add a trait to an entire class, you just want to add a trait to an object instance when the object is created.

Solution

Add the trait to the object when you construct it. This is demonstrated in a simple example:

How to define a Scala trait so it can only be subclassed by a certain type

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 8.6, “How to mark a Scala trait so it can only be subclassed by a certain type.”

Problem

You want to mark your trait so it can only be used by types that extend a given base type.

Solution

To make sure a trait named MyTrait can only be mixed into a class that is a subclass of a type named BaseType, begin your trait with a this: BaseType => declaration, as shown here: