Question

I’m learning Scala and was curious if it would be possible to:

1. Create an object which implements a Java interface in Scala
2. Compile the object into a class file and jar it
3. Use the object from Java

I want to implement a custom lucene query parser in scala and be able to let others access it from a java application.

Answer 1

I’m assuming that by “object” you actually mean “class”. In any case, the answer is yes, you can do this. You will need to take advantage of the Scala/Java joint compiler if you want this all in the same project. For example:

public interface Parser {
    public TokenIterator parse(InputStream is);
}

Then in Scala:

class ParserImpl extends Parser {
  def parse(is: InputStream) = ...
}

Finally, in Java again:

public class Consumer {
    public static void main(String[] args) {
        Parser p = new ParserImpl();       // just like a Java class!
        ...
    }
}

If all of these source files are in the same project, then you will want to compile them using the following invocation of commands:

$ scalac *.scala *.java
$ javac -classpath . *.java

The first command invokes the Scala/Java joint compiler, which will compile the Scala sources and just enough of the Java sources to satisfy any dependencies. The second command invokes the Java compiler with the recently-compiled Scala classes on the classpath.