CustomConverionSerice - no compiler warnings :)

gistfile1.java

    @Override
    public <SOURCE, TARGET> Collection<TARGET> convert(Collection<SOURCE> sources, Class<TARGET> targetClass) {
        final List<TARGET> result = new ArrayList<>();

        if (sources != null) {
            for (SOURCE source : sources) {
                result.add(super.convert(source, targetClass));
            }
        }

        return result;
    }