Using Checkstyle to force the use of Java 8 lambdas

I got asked today whether it was possible to use Checkstyle to force the use of Java 8 lambdas. I can see the merit in wanting to do this.

There does not appear to be an existing check for doing it. However, it is possible using the ever powerful DescendantToken. Just add the following module definition to your Checkstyle configuration to report on code that should be using Java 8 lambdas:

<module name="DescendantToken">
  <property name="tokens" value="METHOD_CALL"/>
  <property name="limitedTokens" value="METHOD_DEF"/>
  <property name="maximumNumber" value="0"/>
  <property name="maximumMessage" value="Use Java 8 Lambdas!"/>
</module>

The definition above will find any method call that contains a method definition, which would be part of an anonymous class definition. Simple but effective.