Scala Cookbook

6.1. Object Casting

Problem

You need to cast an instance of a class from one type to another, such as when creating objects dynamically.

Solution

Use the asInstanceOf method to cast an instance to the desired type. In the following example, the object returned by the lookup method is cast to an instance of a class named Recognizer:

val recognizer = cm.lookup("recognizer").asInstanceOf[Recognizer]

This Scala code is equivalent to the following Java code:

Recognizer recognizer = (Recognizer)cm.lookup("recognizer");

The asInstanceOf method is defined in the Scala Any class and is therefore available on all objects.

Discussion

In dynamic programming, it’s often necessary to cast from one type to another. This approach is needed when using the Spring Framework and instantiating beans from an application context file:

// open/read the application context file
val ctx = new ClassPathXmlApplicationContext("applicationContext.xml")

// instantiate our dog and cat objects from the application context
val dog = ctx.getBean("dog").asInstanceOf[Animal]
val cat = ctx.getBean("cat").asInstanceOf[Animal]

It’s used when reading a YAML configuration file:

val yaml = new Yaml(new Constructor(classOf[EmailAccount]))
val emailAccount = yaml.load(text).asInstanceOf[EmailAccount]

The example shown in the Solution comes from code I wrote to work with an open source Java speech recognition library named Sphinx-4. With this library, many properties are defined in an XML file, and then you create recognizer and microphone ...