Thank you, I´ll look at it :) Although I think I understand the visibility and accsess modifiers in Java, I just didn´t know it´s possible to declare more classes in one file and the rules around that.

So the solution was just to remove the public modifier :) Well, at least I´ve learned something new about java.

Thanks for your suggestion. It would be quite strange solution for java... but I´m gonna try it.

./src/main/java/Leetspeak.java:31: error: class Encoder is public, should be declared in a file named Encoder.java
public abstract class Encoder {
^
./src/main/java/Leetspeak.java:35: error: interface Dictionary is public, should be declared in a file named Dictionary.java
public interface Dictionary extends Map<String, String> {
^
./src/main/java/Leetspeak.java:39: error: class LeetspeakDictionary is public, should be declared in a file named LeetspeakDictionary.java
public class LeetspeakDictionary implements Dictionary {
^
3 errors

I really tried to make an OOP exercise out of this kata. And of course, in my IDE every class has its own file. But how am I supposed to submit my result here correctly?

Thank you! Next time I am going to try every possible digit :))
One more thing, is there some way to run the code within the app? Or just the tests so I need to use another IDE for debugging?