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.”


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


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:

// java
public interface Animal {
    public void speak();

public interface Wagging {
    public void wag();

public interface Running {
    public void run();

you can create a Dog class in Scala with the usual extends and with keywords, just as though you were using traits:

// scala
class Dog extends Animal with Wagging with Running {
    def speak { println("Woof") }
    def wag { println("Tail is wagging!") }
    def run { println("I'm running!") }

The difference is that Java interfaces don’t implement behavior, so if you’re defining a class that extends a Java interface, you’ll need to implement the methods, or declare the class abstract.

The Scala Cookbook

This tutorial is sponsored by the Scala Cookbook, which I wrote for O’Reilly:

You can find the Scala Cookbook at these locations:

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