Scala Cookbook

8.9. Extending a Java Interface Like a 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:

// 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.