What is the accessDeclaredMembers error?

[ninesalt]

I'm trying to write an plugin in for ES but I keep getting this annoying error whenever I try it and I don't know what's causing it. Below is the class that appears in the errors:

public class SentimentAnalyzer {

GoSentiment gs;
Map<Integer, String> lookup;

public interface GoSentiment extends Library {

    class GoString extends Structure {  
        public static class ByValue extends GoString implements Structure.ByValue {}
        public String p;
        public long n;
        protected List<String> getFieldOrder(){
            return Arrays.asList(new String[]{"p","n"});
        }
    }

    int ClassifySentiment(GoString.ByValue str);
}

public SentimentAnalyzer(){
    String path = "/usr/share/elasticsearch/config/ingest-opennlp/sentiment.so";
    gs = Native.loadLibrary(path, GoSentiment.class);
    lookup = new HashMap<>();
    lookup.put(0, "negative");
    lookup.put(1, "positive");
}

public int analyzeSentiment(String text){
    GoSentiment.GoString.ByValue str = new GoSentiment.GoString.ByValue();
    str.p = text;
    str.n = text.length();
    return gs.ClassifySentiment(str);
}

public String toSentimentWord(int value){
    if(lookup.containsKey(value)){
        return lookup.get(value);
    }
    return "null";
}

}

Specifically, the error points to the GoString class declaration. What is happening here?