Question

我试图通过网站上的json加载来填充gridlayout，它必须在点击时显示图片，名称和链接。

当尝试使用asynctask填充时，我收到以下错误：

03-14 15:02:11.053: E/AndroidRuntime(10155): FATAL EXCEPTION: main
03-14 15:02:11.053: E/AndroidRuntime(10155): java.lang.NullPointerException
03-14 15:02:11.053: E/AndroidRuntime(10155):    at com.videotrafico.MainActivity$DownloadCAMARAS.onPostExecute(MainActivity.java:515)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at com.videotrafico.MainActivity$DownloadCAMARAS.onPostExecute(MainActivity.java:1)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at android.os.AsyncTask.finish(AsyncTask.java:631)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at android.os.Looper.loop(Looper.java:137)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at android.app.ActivityThread.main(ActivityThread.java:5227)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at java.lang.reflect.Method.invokeNative(Native Method)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at java.lang.reflect.Method.invoke(Method.java:511)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
03-14 15:02:11.053: E/AndroidRuntime(10155):    at dalvik.system.NativeStart.main(Native Method)

我看到了网页的输出，没问题。

这是我的代码：

public class DownloadCAMARAS extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Create a progressdialog
        mProgressDialog = new ProgressDialog(MainActivity.this);
        // Set progressdialog message
        mProgressDialog.setMessage("Cargando cámaras, esto puede tomar hasta 20 segundos dependiendo de su conexión.");
        mProgressDialog.setIndeterminate(false);
        // Show progressdialog
        mProgressDialog.show();

    }

    @Override
    protected Void doInBackground(Void... params) {
        // Create an array
        arraylist = new ArrayList<HashMap<String, String>>();
        // Retrieve JSON Objects from the given URL address
        jsonobject = JSONfunctions
                .getJSONfromURL("http://www.videotrafico.com/api/camaras.php");

        try {
            // Locate the array name in JSON
            jsonarray = jsonobject.getJSONArray("datos");

            for (int i = 0; i < jsonarray.length(); i++) {
                HashMap<String, String> map = new HashMap<String, String>();
                jsonobject  = jsonarray.getJSONObject(i);

                // Retrive JSON Objects
                map.put("imagen", jsonobject.getString("imagen"));
                map.put("nombrecam", jsonobject.getString("nombrecam"));
                map.put("enlace", jsonobject.getString("enlace"));
                arraylist.add(map);
            }
        } catch (JSONException e) {
            Log.e("Error", e.getMessage());
            e.printStackTrace();
        }
        return null;
    }

    @Override
    protected void onPostExecute(Void args) {


        gridview = (GridView) findViewById(R.id.gridcams);

        adapter2 = new ListViewAdapter2(MainActivity.this, arraylist);

        // Set the adapter to the ListView
        gridview.setAdapter(adapter2);

        // Close the progressdialog
        mProgressDialog.dismiss();
    }
}

现在它给了我这个问题：

我使用的是android支持库v7：

03-14 15:43:03.039: E/AndroidRuntime(11642): FATAL EXCEPTION: main
03-14 15:43:03.039: E/AndroidRuntime(11642): java.lang.ClassCastException: android.support.v7.widget.GridLayout cannot be cast to android.widget.GridView
03-14 15:43:03.039: E/AndroidRuntime(11642):    at com.videotrafico.MainActivity$DownloadCAMARAS.onPostExecute(MainActivity.java:510)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at com.videotrafico.MainActivity$DownloadCAMARAS.onPostExecute(MainActivity.java:1)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at android.os.AsyncTask.finish(AsyncTask.java:631)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at android.os.AsyncTask.access$600(AsyncTask.java:177)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:644)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at android.os.Looper.loop(Looper.java:137)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at android.app.ActivityThread.main(ActivityThread.java:5227)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at java.lang.reflect.Method.invokeNative(Native Method)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at java.lang.reflect.Method.invoke(Method.java:511)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:795)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:562)
03-14 15:43:03.039: E/AndroidRuntime(11642):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

你的第二个问题是：

 gridview = (GridView) findViewById(R.id.gridcams);

返回的视图findViewById(R.id.gridcams)似乎是GridLayout而不是GridView。

==＆GT; ClassCastException异常

解决方案：将ID为R.id.gridcams的视图更改为布局中的GridView，以便findViewById返回GridView。

在执行AsyncTask Android时出现空指针异常

1 个答案: