Scala Cookbook

5.2. Calling a Method on a Superclass

Problem

To keep your code DRY (“Don’t Repeat Yourself”), you want to invoke a method that’s already defined in a parent class or trait.

Solution

In the basic use case, the syntax to invoke a method in an immediate parent class is the same as Java: Use super to refer to the parent class, and then provide the method name. The following Android method (written in Scala) demonstrates how to call a method named onCreate that’s defined in the Activity parent class:

class WelcomeActivity extends Activity {
  override def onCreate(bundle: Bundle) {
    super.onCreate(bundle)
    // more code here ...
  }
}

As with Java, you can call multiple superclass methods if necessary:

class FourLeggedAnimal {
  def walk { println("I'm walking") }
  def run { println("I'm running") }
}

class Dog extends FourLeggedAnimal {
  def walkThenRun {
    super.walk
    super.run
  }
}

Running this code in the Scala REPL yields:

scala> val suka = new Dog
suka: Dog = Dog@239bf795

scala> suka.walkThenRun
I'm walking
I'm running

Controlling which trait you call a method from

If your class inherits from multiple traits, and those traits implement the same method, you can select not only a method name, but also a trait name when invoking a method using super. For instance, given this class hierarchy:

trait Human {
  def hello = "the Human trait"
}

trait Mother extends Human {
  override def hello = "Mother"
}

trait Father extends Human {
  override def hello = "Father"
}

The following code shows different ways to invoke ...