Reading JSON from file using GSON in Android Studio

Question

I'm trying to read JSON from an internal storage file into a list of objects.

My code for reading the file and GSON is:

fis = openFileInput(filename);

BufferedReader reader = new BufferedReader(new InputStreamReader(fis));

StringBuilder data = new StringBuilder();
String line = null;

line = reader.readLine();

while (line != null)
{
     data.append(line).append("\n");
}

data.toString();

reader.close();
fis.close();

Type walletListType = new TypeToken<ArrayList<WalletClass>>(){}.getType();
walletList.add(new Gson().fromJson(data, walletListType));

However, I'm getting the error

Cannot resolve method fromJson('java.lang.stringBuilder, java.lang.reflect.Type')

The JSON I'm trying to load is (it's inside the square brackets because I've serialized it from a list of objects):

[
   {"balance":258,"walletName":"wallet 1"},
   {"balance":5222,"walletName":"wallet 2"},
   {"balance":1,"walletName":"wallet 3"}
]

I know a common fix for this is changing the import code from org to com, however I've already made sure it is com.

@Kirguduck That part works fine. But putting the reader inside the while loop doesn't change anything about the GSON error — Brady Harper
– Brady Harper, Commented Mar 16, 2020 at 17:00
if you use line = reader.readLine(); out of loop then you will read only first line of your json. add logs and check - what do you read. Now you read only part of json and as it appears this is cause of error — Kirguduck
– Kirguduck, Commented Mar 16, 2020 at 17:08

alainlompo · Accepted Answer · 2020-03-16 17:10:45Z

1

the Gson provide a lot of overloads for the fromJson method, here are their signatures:

But as you can see none of them takes the StringBuilder as first argument. That is what the compiler is complaining about. Instead you have constructors that take a String as first argument.

So replace this line:

walletList.add(new Gson().fromJson(data, walletListType));

with:

walletList.add(new Gson().fromJson(data.toString(), walletListType));

And you should be good to go.

answered Mar 16, 2020 at 17:10

alainlompo

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3 Comments

Brady Harper Over a year ago

Yeah I tried that and it didn't work. I already have: data.toString();

alainlompo Over a year ago

What error message do you get when you use data.toString()? Obviously not the same, right?

Brady Harper Over a year ago

My mistake, it did solve it. I assumed that having data.toString() outside of the fromJson method it would still work however it doesn't for some reason. Cheers

Arrowsome · Accepted Answer · 2020-03-16 17:18:21Z

You can use GSON's Type adapter to read and write files. I have written this in kotlin, I hope it helps you.

val builder = GsonBuilder()
      builder.registerTypeAdapter(YourData::class.java, MyTypeAdapter())
      return builder.create()

Sample Type Adapter

class MyTypeAdapter : TypeAdapter<YourData>() {

  @Throws(IOException::class)
  override fun read(reader: JsonReader): YourData {
    var element1
    var element2
    reader.beginObject()
    while (reader.hasNext()) {
      when (reader.nextName()) {
        "element1" -> latitude = reader.nextDouble()
        "element2" -> dropMessage = reader.nextString()
      }
    }
    reader.endObject()

    return YourData(element1, element2)
  }

  @Throws(IOException::class)
  override fun write(out: JsonWriter, yourData: YourData) {
    out.beginObject()
    out.name("element1").value(yourData)
    out.name("element2").value(yourData)
    out.endObject()
  }
}

Usage (Read & Write)

fun saveData(yourData: YourData) {
    val string = gson.toJson(yourData)
    try {
      val dataStream = dataOutputStream(yourData)
      yourStream.write(string.toByteArray())
      yourStream.close()
    } catch (e: IOException) {
      Log.e("FileRepository", "Error")
    }
  }

fun getData(): List<YourData> {
    val data = mutableListOf<YourData>()

    try {
      val fileList = dataDirectory().list()

      fileList.map { convertStreamToString(YourDataInputStream(it)) }.mapTo(data) {
        gson.fromJson(it, YourData::class.java)
      }
    } catch (e: IOException) {
      Log.e("FileRepository", "Error")
    }

    return data
  }

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