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我做我的第一個應用程序，顯示匯率列表。問題是，當我嘗試啓動應用程序，我有一個錯誤。我覺得參數之一，而我通過什麼== null，但I'dont知道哪一個Json在listView中解析 - 一個空對象引用錯誤
錯誤：
E/EGL_emulation: tid 2454: eglSurfaceAttrib(1146): error 0x3009
(EGL_BAD_MATCH)
W/OpenGLRenderer: Failed to set EGL_SWAP_BEHAVIOR on surface 0x9e6cf540,
error=EGL_BAD_MATCH
E/EGL_emulation: tid 2454: eglSurfaceAttrib(1146): error 0x3009
(EGL_BAD_MATCH)
W/OpenGLRenderer: Failed to set EGL_SWAP_BEHAVIOR on surface 0x9e6cf580,
error=EGL_BAD_MATCH
D/AndroidRuntime: Shutting down VM
--------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.lewin.kurswalut, PID: 2266
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:116)
at org.json.JSONTokener.nextValue(JSONTokener.java:94)
at org.json.JSONObject.<init>(JSONObject.java:156)
at org.json.JSONObject.<init>(JSONObject.java:173)
at com.lewin.kurswalut.MainActivity$GetValue.onPostExecute(MainActivity.java:83)
at com.lewin.kurswalut.MainActivity$GetValue.onPostExecute(MainActivity.java:49)
at android.os.AsyncTask.finish(AsyncTask.java:660)
at android.os.AsyncTask.-wrap1(AsyncTask.java)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:677)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
MainActivity.java
package com.lewin.kurswalut;
import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ListAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;
import android.widget.TextView;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.HashMap;
import static com.lewin.kurswalut.R.id.list;
public class MainActivity extends AppCompatActivity {
private ProgressDialog pDialog;
private ListView lv;
ArrayList<HashMap<String, String>> currencyList;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
currencyList = new ArrayList<>();
lv = (ListView) this.findViewById(R.id.list);
new GetValue().execute("http://api.nbp.pl/api/exchangerates/tables/a/?format=json");
}
private class GetValue extends AsyncTask<String, Void, String> {
private ProgressDialog pDialog = new ProgressDialog(MainActivity.this);
@Override
protected void onPreExecute() {
pDialog.setMessage("Please wait...");
pDialog.show();
}
@Override
protected String doInBackground(String... urls) {
try {
URL url = new URL(urls[0]);
URLConnection connection = url.openConnection();
InputStream in = new BufferedInputStream(connection.getInputStream());
return streamConverter(in);
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
protected void onPostExecute(String result) {
pDialog.dismiss();
try {
JSONObject jsonObj = new JSONObject(result);
JSONArray currencyView = jsonObj.getJSONArray("rates");
// looping through All Contacts
for (int i = 0; i < currencyView.length(); i++) {
JSONObject c = currencyView.getJSONObject(i);
String currency = c.getString("currency");
String code = c.getString("code");
String mid = c.getString("mid");
HashMap<String, String> currencyViews = new HashMap<>();
currencyViews.put("currency", currency);
currencyViews.put("code", code);
currencyViews.put("mid", mid);
currencyList.add(currencyViews);
ListAdapter adapter = new SimpleAdapter(
MainActivity.this, currencyList,
R.layout.list_items, new String[]{"currency", "code",
"mid"}, new int[]{R.id.currency,
R.id.code, R.id.mid});
lv.setAdapter(adapter);
}
} catch (JSONException e) {
e.printStackTrace();
}
super.onPostExecute(result);
}
public String streamConverter(InputStream is) {
BufferedReader reader = new BufferedReader(new InputStreamReader(is));
StringBuilder builder = new StringBuilder();
String line = null;
try {
while ((line = reader.readLine()) != null) {
builder.append(line + "\n");
}
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
return builder.toString();
}
}
}
JSON格式：
[
{
"table": "A",
"no": "086/A/NBP/2017",
"effectiveDate": "2017-05-05",
"rates": [
{
"currency": "bat (Tajlandia)",
"code": "THB",
"mid": 0.1110
},
{
"currency": "dolar amerykański",
"code": "USD",
"mid": 3.8490
},
{
"currency": "dolar australijski",
"code": "AUD",
"mid": 2.8471
},
{
"currency": "dolar Hongkongu",
"code": "HKD",
"mid": 0.4945
},
{
"currency": "dolar kanadyjski",
"code": "CAD",
"mid": 2.7963
},
{
"currency": "dolar nowozelandzki",
"code": "NZD",
"mid": 2.6545
},
{
"currency": "dolar singapurski",
"code": "SGD",
"mid": 2.7422
},
{
"currency": "euro",
"code": "EUR",
"mid": 4.2176
},
{
"currency": "forint (Węgry)",
"code": "HUF",
"mid": 0.013511
},
{
"currency": "frank szwajcarski",
"code": "CHF",
"mid": 3.8928
},
{
"currency": "funt szterling",
"code": "GBP",
"mid": 4.9792
},
{
"currency": "hrywna (Ukraina)",
"code": "UAH",
"mid": 0.1452
},
{
"currency": "jen (Japonia)",
"code": "JPY",
"mid": 0.034266
},
{
"currency": "korona czeska",
"code": "CZK",
"mid": 0.1572
},
{
"currency": "korona duńska",
"code": "DKK",
"mid": 0.5671
},
{
"currency": "korona islandzka",
"code": "ISK",
"mid": 0.03628
},
{
"currency": "korona norweska",
"code": "NOK",
"mid": 0.4442
},
{
"currency": "korona szwedzka",
"code": "SEK",
"mid": 0.4353
},
{
"currency": "kuna (Chorwacja)",
"code": "HRK",
"mid": 0.5675
},
{
"currency": "lej rumuński",
"code": "RON",
"mid": 0.9277
},
{
"currency": "lew (Bułgaria)",
"code": "BGN",
"mid": 2.1565
}
]
}
]
我還有一個問題：我如何獲得選定的項目，例如其中上面像「代碼」：「美元」，「歐元」，「挪威克朗」？
謝謝你的建議
'new JSONObject（result）;'...'result'爲null –
'doInBackground'進入'return null;'因爲還有一些其他錯誤......就像您不允許在您的應用程序中使用Internet權限 –