Question

i am attempting to turn a json string into objects with gson.

I have a very simple example below, and it runs, but the resulting answer is empty, ie: my Answer objects’s text field is empty.

import com.google.gson.*;

public class Meow {

    public static void main(String[] args) throws Exception{

        Gson gson = new Gson();
        String jsonOutput = "[{\"answer\":{\"text\":\"text1\"}},{\"answer\":{\"text\":\"text2\"}} ]";

        Answer[] a = gson.fromJson(jsonOutput, Answer[].class);

        for(Answer i:a) {
          System.out.println(i.text);
        }       
    }

    public class Answer {

        public String text;

        public Answer(String text) {
            super();
            this.text=text;
        }

        public String toString(){
            return text;
        }

        public void setText(String a){
            this.text=a;
        }
    }

}

Answer 1

Because your JSON doesn’t match your class.

Your JSON right now is an array of objects, each containing an answer object as a field.

Your JSON the way you have things would need to look like:

String jsonOutput = "[{\"text\":\"text1\"},{\"text\":\"text2\"}]";

Edit to add from comments:

If you can’t change the output, you need a “wrapper”. Something like:

public class AnswerWrapper {
    public Answer answer;

    // etc
}

And use an array of those. That is what the JSON will map to. It can’t see them as Answer objects because … they’re not.

One More Edit to Add: Your other option is to write custom deserializers for your classes. I’m a bit mixed on whether you should do this or not, but it will work. The reason I say that is that you have JSON that isn’t an array of Answer objects, but you want it to be. I think I’d be annoyed if I came across this in production code because without understanding what was going on it could be confusing.

With that caveat being said, you can create a custom JsonDeserializer and use GsonBuilder:

class AnswerDeserializer implements JsonDeserializer<Answer> {

    public Answer deserialize(JsonElement je, Type type, 
                              JsonDeserializationContext jdc) 
                                   throws JsonParseException {

        return new Answer(je.getAsJsonObject().get("answer")
                            .getAsJsonObject().get("text").getAsString());
    }

}

Then your code would look like:

public static void main(String[] args) throws Exception{

    String jsonOutput = "[{\"answer\":{\"text\":\"text1\"}},{\"answer\":{\"text\":\"text2\"}} ]";

    GsonBuilder gsonBuilder = new GsonBuilder();
    gsonBuilder.registerTypeAdapter(Answer.class, new AnswerDeserializer());

    Gson gson = gsonBuilder.create();

    Answer[] a = gson.fromJson(jsonOutput, Answer[].class);

    for(Answer i:a) {
        System.out.println(i.text);
    }       
}

If it were me, and I had JSON that wasn’t what I needed it to be but wanted to use GSON to directly serialize/deserialize I’d create the Answer class as a wrapper that hid the details:

/**
 *  Due to how our JSON is being provided we created an inner
 *  class. 
 **/ 
public class Answer {

    private RealAnswer answer;

    private class RealAnswer {

        public String text;
    }

    ...
}

With the public getters/setters for Answer accessing the private RealAnswer. It just seems way cleaner and easier to understand for the next guy.