Scala Cookbook

5.8. Declaring That a Method Can Throw an Exception

Problem

You want to declare that a method can throw an exception, either to alert callers to this fact or because your method will be called from Java code.

Solution

Use the @throws annotation to declare the exception(s) that can be thrown. To declare that one exception can be thrown, place the annotation just before the method signature:

@throws(classOf[Exception])
override def play {
  // exception throwing code here ...
}

To indicate that a method can throw multiple exceptions, list them all before the method signature:

@throws(classOf[IOException])
@throws(classOf[LineUnavailableException])
@throws(classOf[UnsupportedAudioFileException])
def playSoundFileWithJavaAudio {
  // exception throwing code here ...
}

Discussion

The two examples shown are from an open source project I created that lets developers play WAV, AIFF, MP3, and other types of sound files. I declared that these two methods can throw exceptions for two reasons. First, whether the consumers are using Scala or Java, if they’re writing robust code, they’ll want to know that something failed.

Second, if they’re using Java, the @throws annotation is the Scala way of providing the throws method signature to Java consumers. It’s equivalent to declaring that a method throws an exception with this Java syntax:

public void play() throws FooException {
  // code here ...
}

It’s important to note that Scala’s philosophy regarding checked exceptions is different than Java’s. Scala doesn’t ...