Question

我正在尝试将一些JSONObject数据从我的AsyncTask.OnPostExecute传递回我的MainActivity.JSONCallBackComplete。

我的问题是当我尝试检查传回的JSONObject时，我得到以下的执行：

10-14 18:40:25.820: E/AndroidRuntime(4153): FATAL EXCEPTION: main
10-14 18:40:25.820: E/AndroidRuntime(4153): java.lang.NullPointerException
10-14 18:40:25.820: E/AndroidRuntime(4153):     at com.icerge.revivaltimes.MainActivity.JSONCallBackComplete(MainActivity.java:31)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at com.icerge.revivaltimes.JsonObj.onPostExecute(JsonObj.java:70)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at com.icerge.revivaltimes.JsonObj.onPostExecute(JsonObj.java:1)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at android.os.AsyncTask.finish(AsyncTask.java:417)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at android.os.AsyncTask.access$300(AsyncTask.java:127)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at android.os.Handler.dispatchMessage(Handler.java:99)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at android.os.Looper.loop(Looper.java:144)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at android.app.ActivityThread.main(ActivityThread.java:4937)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at java.lang.reflect.Method.invokeNative(Native Method)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at java.lang.reflect.Method.invoke(Method.java:521)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:858)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
10-14 18:40:25.820: E/AndroidRuntime(4153):     at dalvik.system.NativeStart.main(Native Method)

我的MainActivity类如下：

public class MainActivity extends Activity{

        private JSONObject jsonData = null;

        public void JSONCallBackComplete(JSONObject jsonData){
            this.jsonData = jsonData;
            Log.e("TESTing in callback: ", jsonData.toString()  );
        }

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

            new JsonObj(this).execute("http://myserver/json");
         }
    }

我的AsyncTask类如下：

public class JsonObj extends AsyncTask<String, Void, JSONObject>{
    MainActivity activity;
    int tid;
    String term;

    public JsonObj(MainActivity activity){
        this.activity = activity;
//      Log.e("TESTING: ", activity.getClass().toString());
    }

    @Override
    protected JSONObject doInBackground(String... url) {
        // TODO Auto-generated method stub
        DefaultHttpClient   httpclient = new DefaultHttpClient(new BasicHttpParams());
        HttpPost httppost = new HttpPost(url[0]);
        JSONObject jsonObject = null;
        // Depends on your web service
        httppost.setHeader("Content-type", "application/json");

        InputStream inputStream = null;
        String result = null;
        try {
            HttpResponse response = httpclient.execute(httppost);           
            HttpEntity entity = response.getEntity();

            inputStream = entity.getContent();
            // json is UTF-8 by default
            BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8);
            StringBuilder sb = new StringBuilder();

            String line = null;
            while ((line = reader.readLine()) != null){
                sb.append(line + "\n");
            }
            result = sb.toString();
            Log.e("JSON-Test [RESULT]: ", result);
            jsonObject = new JSONObject(result);
        } catch (Exception e) { 
            Log.e("JSON-Test [exception]: ", e.toString());
        }
        finally {
            try{if(inputStream != null)inputStream.close();}catch(Exception squish){}
        }

        return jsonObject;
    }

    @Override
protected void onPostExecute(JSONObject result) {
    // TODO Auto-generated method stub
    super.onPostExecute(result);
    if( !result.equals(null) ){
        this.activity.JSONCallBackComplete(result);
    }
}
}

我在AsyncTask中有一个构造函数，用于分配MainActivity-Context并使用它来使用JSONObject参数调用我的JSONCallBackComplete反馈函数。

但是，我没有显示我的JSON-DATA，而是获得了上面粘贴的异常。

Answer 1

你有NullPointerException个。我看到的第一个来自这里

if(!result.equals(null) ){

这应该是

if (result != null) {

您不应该使用.equals()与null进行比较，因为.equals()通常用于检查2个对象的人类感知值是否相同，而不是内存地址。如果要将某些内容与null内存地址进行比较，只需使用直接==或!=，其中Java将为您比较地址（您想要的内容）。

Android AsyncTask-to-Activity回调nullPointerException

1 个答案: