JsonReader - reading an array throws Expected a name but was NULL

Question

I'm using JsonReader to parse some JSON data. It's a big array of elements of similar structure:

[  
   {  
      "type":"duel",
      "discipline":"leagueoflegends",
      "tournament_id":"57ee13fe140ba0cd2a8b4593",
      "opponents":[  
         {  
            "participant":null,
            "forfeit":false,
            "number":1,
            "result":1,
            "score":3
         },
         {  
            "participant":null,
            "forfeit":false,
            "number":2,
            "result":3,
            "score":2
         }
      ],
      "id":"58078b2770cb49c45b8b45bf",
      "status":"completed",
      "number":1,
      "stage_number":1,
      "group_number":1,
      "round_number":1,
      "date":"2016-10-01T05:00:00+0300",
      "timezone":"Europe/Helsinki",
      "match_format":null
   },
   ...
]

There's hundreds of similar elements in this array. I only need some data from each element and I wrote code similar to this:

public List<Match> readJsonStream(InputStream in) throws IOException {
    JsonReader reader = new JsonReader(new InputStreamReader(in, "UTF-8"));
    try {
        return readMatchesArray(reader);
    } finally {
        reader.close();
    }
}

public List<Match> readMatchesArray(JsonReader reader) throws IOException {
    List<Match> matches = new ArrayList<>();

    reader.beginArray();
    while (reader.hasNext()) {
        matches.add(readMatch(reader));
    }
    reader.endArray();
    return matches;
}

public Match readMatch(JsonReader reader) throws IOException {
    String status = null, date = null, time = null;
    Match.Bracket bracket = null;
    int id = -1, round = -1;
    Match.Team team1 = null, team2 = null;

    reader.beginObject();
    while (reader.hasNext()) {
        String name = reader.nextName();
        switch (name) {
            case "number":
                id = reader.nextInt();
                break;
            case "status":
                status = reader.nextString().toUpperCase();
                break;
            case "group_number":
                if (reader.nextInt() == 1) {
                    bracket = WINNERS;
                } else if (reader.nextInt() == 2) {
                    bracket = LOSERS;
                }
                break;
            case "round_number":
                round = reader.nextInt();
                break;
            case "date":
                try {
                    String tempDateRaw = reader.nextString();
                    String[] tempDate = tempDateRaw.split("T");
                    String[] tempTime = tempDate[1].split(":");

                    date = tempDate[0];
                    time = tempTime[0] + ":" + tempTime[1];
                } catch (IllegalStateException e) {
                    date = null;
                    time = null;
                }
                break;
            case "opponents":
                int counter = 0;
                reader.beginArray();
                while (reader.hasNext()) {
                    if (counter == 0) {
                        team1 = readTeam(reader);
                    } else {
                        team2 = readTeam(reader);
                    }
                    counter++;
                }
                reader.endArray();
                break;
            default:
                reader.skipValue();
                break;
        }
    }
    reader.endObject();
    return new Match(team1, team2, id, round, bracket, status, date, time);
}


public Match.Team readTeam(JsonReader reader) throws IOException {
    String teamName = null;
    int score = -1;
    boolean winner = false;

    reader.beginObject();
    while (reader.hasNext()) {
            String name = reader.nextName(); //this is where the error occurs
            switch (name) {
                case "participant":
                    if(reader.peek() != JsonToken.NULL) {
                        teamName = reader.nextString();
                    }
                    else {
                        teamName = null;
                    }
                    break;
                case "score":
                    if(reader.peek() != JsonToken.NULL) {
                        score = reader.nextInt();
                    }
                    else {
                        score = -1;
                    }
                    break;
                case "result":
                    if(reader.peek() != JsonToken.NULL) {
                        winner = reader.nextInt() == 1;
                    }
                    else {
                        winner = false;
                    }
                    break;
                default:
                    reader.skipValue();
                    break;
            }
    }
    reader.endObject();

    return new Match.Team(teamName, score, winner);
}

However, when I'm trying to parse the opponents array I have an error in readTeam() Expected a name but was NULL and I'm very confused why is this happening.

At first I didn't know about the peek() so in readMatch() method I used try/catch, I have to change that, but it shouldn't be relevant to the problem. I couldn't find anything about this particular error, there's quite a lot topics regarding other, similar errors (Expected name is string/int/whatever) but here I can't quite find the reason why it's not working. Do you have any ideas how to fix it?

Is there a reason you are not using GSON to parse the JSON? — Neerajlal K
– Neerajlal K, Commented Nov 6, 2016 at 17:13
Well, not really. I'm still beginner in Java and a complete newb in Android, and that was the first thing that I saw regarding parsing JSON. Previously I simply used StringBuilder and used getJSONObject(), but because the file is so big I ran out of memory. I guess I could try GSON, but first of all I don't have too much time and second of all, I'm quite confident that my problem is easily solvable, I just can't find the cause. — paolostyle
– paolostyle, Commented Nov 6, 2016 at 17:22

Miha_x64 · Accepted Answer · 2018-07-11 12:56:31Z

4

You should consume null tokens, too, calling nextNull() or skipValue(), otherwise Reader won't advance and will stick with this null. For example:

case "result":
    if(reader.peek() != JsonToken.NULL) {
        winner = reader.nextInt() == 1;
    } else {
        winner = false;
        reader.nextNull(); // note on this
    }
    break;

https://developer.android.com/reference/android/util/JsonReader

Null literals can be consumed using either nextNull() or skipValue().

answered Jul 11, 2018 at 12:56

Miha_x64

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paolostyle · Accepted Answer · 2016-11-06 18:12:21Z

1

Alright, I found a solution, just like I thought, it was simpler than I expected. So basically when I'm checking for null values, like this:

 if(reader.peek() != JsonToken.NULL) {
   teamName = reader.nextString();
 }
 else {
   teamName = null;
 }

the so called "reader cursor" doesn't move, so it's basically stuck on the value of the teamName (in this case). So the simplest solution is to just put reader.skipValue(); in else block. I also found another bug in my code, in case "group_number" where I actually move the aforementioned "cursor" twice, but the solution is obvious. Hopefully it will be helpful for someone.

answered Nov 6, 2016 at 18:12

paolostyle

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1 Comment

gdawgrancid Over a year ago

This will teach me to not read through all the answers initially :). This was exactly the problem I had. I used nextNull() instead of skipValue(), but it doesn't look like it matters in my situation.

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