@FunctionalInterface
public interface LambdaKey<T> {

    String key(T value);
}

--- different file
public class LambdaKeyConverter<T> {

    public LambdaKeyConverter(final LambdaKey<T> lambdaKey) {
    }
}

--- different file
import java.util.List;

public class SomeClass {
    private final LambdaKeyConverter converter = new LambdaKeyConverter<List>(list -> String.valueOf(list.size()));
}