使用GSON在Android Studio中从文件读取JSON。

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英文:

Reading JSON from file using GSON in Android Studio

问题

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.toString(), 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.

英文:

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(&quot;\n&quot;);
}

data.toString();

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

Type walletListType = new TypeToken&lt;ArrayList&lt;WalletClass&gt;&gt;(){}.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):

[
   {&quot;balance&quot;:258,&quot;walletName&quot;:&quot;wallet 1&quot;},
   {&quot;balance&quot;:5222,&quot;walletName&quot;:&quot;wallet 2&quot;},
   {&quot;balance&quot;:1,&quot;walletName&quot;:&quot;wallet 3&quot;}
]

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.

答案1

得分: 1

以下是翻译好的部分:

the Gson提供了许多fromJson方法的重载,以下是它们的签名:

使用GSON在Android Studio中从文件读取JSON。

但是正如你所见,它们中没有任何一个将StringBuilder作为第一个参数。这就是编译器所抱怨的内容。相反,你有一些以String作为第一个参数的构造函数。

因此,将这行代码:

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

替换为:

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

这样你应该就可以继续了。

英文:

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

使用GSON在Android Studio中从文件读取JSON。

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.

答案2

得分: 0

你可以使用GSON的类型适配器(Type adapter)来读写文件
我已经用Kotlin写了这部分代码希望能对你有所帮助

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

示例类型适配器(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()
  }
}

使用示例(读取和写入)

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
}

<details>
<summary>英文:</summary>

You can use GSON&#39;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 &amp; 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(&quot;FileRepository&quot;, &quot;Error&quot;)
}

return data

}


</details>



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  • 本文由 发表于 2020年3月17日 00:47:11
  • 转载请务必保留本文链接:https://go.coder-hub.com/60709964.html
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