At work, there are a couple of JAX-RS microservices I develop and maintain. Fortunately, all of these services and their consumers are written in Scala. Up until now, I have never needed to access the service APIs from Java. But some other team recently needed to do that! And that was where shit hit the fan. Consider the following ScalaApi class:

class ScalaApi {

  def default: String = "test"

}

Did you notice something wrong with the class methods? While it is perfectly legitimate to use default as a keyword in the JVM byte code, it is not allowed by the Java Language Specification 3.9. (Another slightly related discussion: Reserved words as variable or method names.) Hence, you cannot access it via a simple new ScalaApi().default() call in Java. To the best of my knowledge, your only safe bet becomes using Java’s Reflection API:

import java.lang.reflect.Method;

public class Consumer {

    public static void main(String[] args) {
        ScalaApi scalaApi = new ScalaApi();
        String scalaApiDefault = scalaApiDefault(scalaApi);
        System.out.println("ScalaApi#default: " + scalaApiDefault);
    }

    private static String scalaApiDefault(ScalaApi scalaApi) {
        try {
            Method method = scalaApi.getClass().getDeclaredMethod("default");
            return (String) method.invoke(scalaApi);
        } catch (NoSuchMethodException | InvocationTargetException | IllegalAccessException cause) {
            throw new RuntimeException(cause);
        }
    }

}

This issue raised the following question: Where did I make a mistake? I believe, it is not possible to come up with a universally legitimate API targeting all available languages that compile to JVM byte code. I think that is one of the reasons why programming languages provide constructs to use reserved keywords in the source code, such as reserved in Scala or @reserved in C#, etc.