java - JSON 解析和 NullPointer

Question

我正在尝试解析一个 JSON 对象，但遇到了一个空指针错误，以及一个 Value Cannot of type java.lang.String cannot be converted to JSONObject 错误。我的代码如下。如果有任何帮助，我将不胜感激。

06-30 14:48:18.151: E/JSON Parser(10231): Error parsing data org.json.JSONException: Value Cannot of type java.lang.String cannot be converted to JSONObject
06-30 14:48:18.156: E/AndroidRuntime(10231): FATAL EXCEPTION: main
06-30 14:48:18.156: E/AndroidRuntime(10231): Process: rafa.weatherapp, PID: 10231
06-30 14:48:18.156: E/AndroidRuntime(10231): java.lang.NullPointerException
06-30 14:48:18.156: E/AndroidRuntime(10231):    at rafa.weatherapp.MainActivity$AsyncTaskParseJson.onPostExecute(MainActivity.java:72)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at rafa.weatherapp.MainActivity$AsyncTaskParseJson.onPostExecute(MainActivity.java:1)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at android.os.AsyncTask.finish(AsyncTask.java:632)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at android.os.Handler.dispatchMessage(Handler.java:102)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at android.os.Looper.loop(Looper.java:146)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at android.app.ActivityThread.main(ActivityThread.java:5602)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at java.lang.reflect.Method.invokeNative(Native Method)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at java.lang.reflect.Method.invoke(Method.java:515)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
06-30 14:48:18.156: E/AndroidRuntime(10231):    at dalvik.system.NativeStart.main(Native Method)

这是我的 MainActivity

    package rafa.weatherapp;

import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends Activity {

    TextView textview = null;
    JSONObject currentTemperature = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        textview = (TextView) findViewById(R.id.textview);
        // we will using AsyncTask during parsing 
        new AsyncTaskParseJson().execute();
    }

    // you can make this class as another java file so it will be separated from your main activity.
    public class AsyncTaskParseJson extends AsyncTask<String, String, String> {

        final String TAG = "AsyncTaskParseJson.java";

        // set your json string url here
       String yourJsonStringUrl = "https://api.forecast.io/forecast/5530508d3568e57848d53bf10cfade1f/37.8267,-122.42";

        @Override
        protected void onPreExecute() {}

        @Override
        protected String doInBackground(String... arg0) {

            try {

                // instantiate our json parser
                JSONParser jParser = new JSONParser();

                // get json string from url
                JSONObject json = jParser.getJSONFromUrl(yourJsonStringUrl);

                JSONObject weatherData = new JSONObject();


                currentTemperature = weatherData.getJSONObject("currently");
                currentTemperature.getString("temperature").toString();

                textview = (TextView) findViewById(R.id.textview);
                textview.setText((CharSequence) currentTemperature);
                Toast.makeText(MainActivity.this, "Text set", Toast.LENGTH_LONG);




            } catch (JSONException e) {
                e.printStackTrace();
            }

            return null;
        }

        @Override
        protected void onPostExecute(String strFromDoInBg) {
            currentTemperature.toString();
             textview.setText((CharSequence) (currentTemperature));
        }
    }
}

Answer 1

我看到几个问题:

• 首先你得到一个空指针，因为你返回 null来自 doInBackground()
• 您正尝试在 doInBackground 中显示 toast，这在后台线程中是不允许的
• 您应该在 onCreate 方法中初始化您的 textView。
• 您正在尝试从 weatherData 中获取您从未使用 json 初始化的数据。

---

public class AsyncTaskParseJson extends AsyncTask<String, String, String> {
    String yourJsonStringUrl = "https://api.forecast.io/forecast/5530508d3568e57848d53bf10cfade1f/37.8267,-122.42";
    @Override
    protected String doInBackground(String... url) {
      String temperature = "";
        try {

            // instantiate our json parser
            JSONParser jParser = new JSONParser();
            // get json string from url
            JSONObject json = jParser.getJSONFromUrl(yourJsonStringUrl);
            JSONObject currentData = json.getJSONObject("currently");
            temperature = currentTemperature.getString("temperature");    

        } catch (JSONException e) {
            e.printStackTrace();
        }

        return temperature;
    }

    @Override
    protected void onPostExecute(String currentTemp) {
         textview.setText((CharSequence) (currentTemp));
    }
}

当我再次阅读错误时:

解析数据时出错 org.json.JSONException: Value Cannot of type java.lang.String cannot be converted to JSONObject

似乎 jParser.getJSONFromUrl() 没有返回 JSONObject 而是一个字符串，在这种情况下，您应该将字符串传递给 JSONObject构造函数:

 String jsonString = jParser.getJSONFromUrl(yourJsonStringUrl);
 JSONObject json = new JSONObject(jsonString);

但是你写了这个函数，只有你知道它返回什么。