listView中的JSON解析 - 空对象参考错误

Question

i执行第一个显示汇率列表的应用程序。proble是当我尝试启动应用程序时，我有一个错误。我认为我经过的参数之一== null，但我知道哪个

错误：

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
      }
    ]
  }
]

我还有另一个问题：如何获得选定的项目，例如上面是"代码"：" USD"，" EUR"，" NOK"？

谢谢您的建议

Answer 1

我相信您是错误的。这是一个JSON数组，然后在您内部有一个JSON对象，然后rates是JSON数组

Answer 2

方法onpostexecute获取null，然后在创建新的jsonobject(结果(时捕获异常。