Unable to start activity ComponentInfo: java.lang.NullPointerException error while try to create the numberpicker Android

Question

您好，在我的应用程序中，我正在尝试创建 NumberPicker，但它无法正常工作，出现类似错误。

    06-12 13:07:33.938: E/AndroidRuntime(7521): FATAL EXCEPTION: main
    06-12 13:07:33.938: E/AndroidRuntime(7521): Process: com.politicalmileage.elangovan, PID: 7521
    06-12 13:07:33.938: E/AndroidRuntime(7521): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.politicalmileage.elangovan/com.politicalmileage.elangovan.Schedule}: java.lang.NullPointerException
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.os.Handler.dispatchMessage(Handler.java:102)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.os.Looper.loop(Looper.java:136)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.ActivityThread.main(ActivityThread.java:5017)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at java.lang.reflect.Method.invokeNative(Native Method)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at java.lang.reflect.Method.invoke(Method.java:515)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at dalvik.system.NativeStart.main(Native Method)
    06-12 13:07:33.938: E/AndroidRuntime(7521): Caused by: java.lang.NullPointerException
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at com.politicalmileage.elangovan.Schedule.onCreate(Schedule.java:58)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.Activity.performCreate(Activity.java:5231)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
    06-12 13:07:33.938: E/AndroidRuntime(7521):     ... 11 more

我的 XML 代码。

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >

<TextView
    android:id="@+id/title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:textColor="#43bd00"
    android:textSize="16sp"
    android:textStyle="bold"
    android:text="TextView" />


<TextView
    android:id="@+id/cost"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/title"
    android:text="TextView" />



<Button
    android:id="@+id/bAdd"
    style="?android:attr/buttonStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/title"
    android:layout_marginLeft="17dp"
    android:layout_toRightOf="@+id/title"
    android:text="+" />

<Button
    android:id="@+id/bSub"
    style="?android:attr/buttonStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/bAdd"
    android:layout_toRightOf="@+id/tvDisplay"
    android:text="-" />

<TextView
    android:id="@+id/tvDisplay"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBaseline="@+id/bSub"
    android:layout_alignBottom="@+id/bSub"
    android:layout_toRightOf="@+id/bAdd"
    android:text="1" />

我的 java 代码。

   public class Schedule extends ListActivity {

     private ProgressDialog pDialog;
      int counter = 1;
      Button add,sub;
      TextView display;

// URL to get contacts JSON
private static String url = "myurl";

// JSON Node names
private static final String TAG_SCHEDULES = "schedule";
private static final String TAG_ID = "id";
private static final String TAG_TITLE = "title";
/*private static final String TAG_CONTENT = "content";*/
private static final String TAG_COST = "cost";
/*private static final String TAG_DATETIME = "date_time";*/

// contacts JSONArray
JSONArray schedules = null;

// Hashmap for ListView
ArrayList<HashMap<String, String>> schedule_list;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.schedule_list_view);
    add = (Button) findViewById(R.id.bAdd);
    sub = (Button) findViewById(R.id.bSub);
    display = (TextView) findViewById(R.id.tvDisplay);
    add.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
        counter++;
        display.setText( "" + counter);
        }
    });

    sub.setOnClickListener(new View.OnClickListener() {

        public void onClick(View v) {
        counter--;
        display.setText( "" + counter);
        }
    });


    schedule_list = new ArrayList<HashMap<String, String>>();

    ListView lv = getListView();





    // Listview on item click listener
    lv.setOnItemClickListener(new OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                int position, long id) {
            // getting values from selected ListItem
            String title = ((TextView) view.findViewById(R.id.title)).getText().toString();
            /*String content = ((TextView) view.findViewById(R.id.content)).getText().toString();*/
            String cost = ((TextView) view.findViewById(R.id.cost)).getText().toString();
            /*String datetime = ((TextView) view.findViewById(R.id.datetime)).getText().toString();*/



            // Starting single contact activity
            Intent in = new Intent(getApplicationContext(), ScheduleDetail.class);
            in.putExtra(TAG_TITLE, title);
            in.putExtra(TAG_COST, cost);
            /*in.putExtra(TAG_CONTENT, content);
            in.putExtra(TAG_DATETIME, datetime);*/
            startActivity(in);

        }
    });

    // Calling async task to get json
    new GetSchedule().execute();
}

/**
 * Async task class to get json by making HTTP call
 * */
private class GetSchedule extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Showing progress dialog
        pDialog = new ProgressDialog(Schedule.this);
        pDialog.setMessage("Please wait...");
        pDialog.setCancelable(true);
        pDialog.show();

    }

    @Override
    protected Void doInBackground(Void... arg0) {
        // Creating service handler class instance
        ServiceHandler sh = new ServiceHandler();

        // Making a request to url and getting response
        String jsonStr = sh.makeServiceCall(url, ServiceHandler.GET);

        Log.d("Response: ", "> " + jsonStr);

        if (jsonStr != null) {
            try {
                JSONObject jsonObj = new JSONObject(jsonStr);

                // Getting JSON Array node
                schedules = jsonObj.getJSONArray(TAG_SCHEDULES);

                // looping through All Schdules
                for (int i = 0; i < schedules.length(); i++) {
                    JSONObject c = schedules.getJSONObject(i);

                    String id = c.getString(TAG_ID);
                    String title = c.getString(TAG_TITLE);
                    /*String content = c.getString(TAG_CONTENT);*/
                    String cost = c.getString(TAG_COST);
                    /*String datetime = c.getString(TAG_DATETIME);*/

                    // Phone node is JSON Object

                    // tmp hashmap for Schedule
                    HashMap<String, String> schedule_file = new HashMap<String, String>();

                    // adding each child node to HashMap key => value
                    schedule_file.put(TAG_ID, id);
                    schedule_file.put(TAG_TITLE, title);
                    /*schedule_file.put(TAG_CONTENT, content);*/
                    schedule_file.put(TAG_COST, cost);
                    /*schedule_file.put(TAG_DATETIME, datetime);*/

                    // adding schedule list
                    schedule_list.add(schedule_file);
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        } else {
            Log.e("ServiceHandler", "Couldn't get any data from the url");
        }

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        super.onPostExecute(result);
        // Dismiss the progress dialog
        if (pDialog.isShowing())
            pDialog.dismiss();
        /**
         * Updating parsed JSON data into ListView
         * */

        ListAdapter adapter = new SimpleAdapter(
                Schedule.this, schedule_list, 
                R.layout.schedule_list_item, 
                new String[] { TAG_TITLE, TAG_COST }, 
                new int[] { R.id.title,R.id.cost });

        setListAdapter(adapter);
      }
     }
   }

我正在尝试使用我的服务器的一些文本创建最小最大数，它无法正常工作，它显示空指针异常错误请在上面的代码中告诉我在哪里做错了如何解决这个问题。

谢谢。

Answer 1

您使用setContentView() 设置的schedule_list_view 布局不包含您尝试使用findViewById() 访问的视图，并且返回null。在 null 上调用方法会导致 NPE。

如果schedule_list_view 是内容视图，由于缺少android:id/list ListView，您会在getListView() 得到不同的异常。