interface

What def, val, and var fields in Scala traits look like after they’re compiled (including the classes that extend them) alvin April 14, 2019 - 6:05pm
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.

A Java Factory Pattern (Factory method) example

Java Design Patterns FAQ: Can you provide an example of the Factory Pattern in Java?

Sure. In this article we'll look at a small-but-complete example of the Factory Pattern ("Factory Design Pattern") implemented in Java.

How to redesign the Twitter user interface (UI)

Twitter UI redesign: After using Twitter for the last several months, it seems like their user interface is out of sync with how I want to use their data. As I was waiting for dinner to cook last night I spent a few minutes thinking about the Twitter user interface (UI) and how I might redesign it. Here are my thoughts.

The current Twitter web interface

The things the Twitter web interface does well currently are:

How to wrap Scala traits so they can be used from Java code

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 17.7, “How to write Scala traits so they can be used from Java code.”

Problem

You’ve written a Scala trait with implemented methods and need to be able to use those methods from a Java application.

Solution

You can’t use the implemented methods of a Scala trait from Java, so wrap the trait in a class.

How to extend a Java interface like a Scala trait

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is a very short recipe, Recipe 8.9, “How to extend a Java interface like a Scala trait.”

Problem

You want to implement a Java interface in a Scala application.

Solution

In your Scala application, use the extends and with keywords to implement your Java interfaces, just as though they were Scala traits.

Given these three Java interfaces:

How to declare that a Scala trait can only be mixed into a type that has a specific method

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 8.7, “How to declare that a Scala trait can only be mixed into a type that has a specific method.”

Problem

You only want to allow a trait to be mixed into a type (class, abstract class, or trait) that has a method with a given signature.

Solution

Use a variation of the self-type syntax that lets you declare that any class that attempts to mix in the trait must implement the method you specify.

Scala: How to limit which classes can use a trait by inheritance

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 8.5, “How to limit which classes can use a trait by inheritance.”

Problem

You want to limit a trait so it can only be added to classes that extend a superclass or another trait.

How to use Scala traits as simple mixins

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 8.4, “How to use Scala traits as simple mixins (or, How to mix in Scala traits).”

Problem

You want to design a solution where multiple traits can be mixed into a class to provide a robust design.

Solution

To implement a simple mixin, define the methods you want in your trait, then add the trait to your class using extends or with. For instance, the following code defines a Tail trait:

How to use a Scala trait like an abstract class

This is an excerpt from the Scala Cookbook (partially modified for the internet). This is a short recipe, Recipe 8.3, “How to use a Scala trait like an abstract class.”

Problem

You want to use a trait as something like an abstract class in Java.

Solution

Define methods in your trait just like regular Scala methods. In the class that extends the trait, you can override those methods or use them as they are defined in the trait.