从其他类调用方法给出 NullPointerException答案

【问题标题】：Calling method from other class give NullPointerException从其他类调用方法给出 NullPointerException
【发布时间】：2015-02-23 18:02:46
【问题描述】：

我有一个活动类，我从那里调用一个ListView 适配器。在适配器类中，我为每行中的两个按钮提供了 onClick 方法。当我单击一个按钮时，我从数据库中删除了该行，但我必须调用我的 Activity 类中的方法，以便更新 UI，但是像这样调用这个方法：

MainActivity main = new MainActivity();
main.removeTimer();

抛出NullPointerException。这是我在 Activity 类中的方法：

public void removeTimer() {
    Cursor result1 = myDb.rawQuery("SELECT * from Timers", null);
    result1.moveToFirst();
    timers.clear();
    names.clear();
    while (result1.isAfterLast() == false) {
        timers.add(result1.getString(result1.getColumnIndex(TIME_COLUMN)));
        names.add(result1.getString(result1.getColumnIndex(NAME_COLUMN)));
        result1.moveToNext();
    }
    timerRow.clear();
    for (int i = 0; i < timers.size(); i++) {
        TimerRow item = new TimerRow(timers.get(i), names.get(i), bRemove,
                bStartStop);
        timerRow.add(item);
    }
    adapter.notifyDataSetChanged();
}

如果我在 Activity 类中调用它，它工作得非常好。这就是我从ListViewAdapter 类中调用该方法的方式：

public View getView(int position, View convertView, ViewGroup parent) {
    final ViewHolder holder;
    final TimerRow rowItem = getItem(position);

    LayoutInflater mInflater = (LayoutInflater) context
            .getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
    if (convertView == null) {
        convertView = mInflater.inflate(R.layout.timer_row, null);
        holder = new ViewHolder();
        // make layout params
        holder.txtTimer = (TextView) convertView.findViewById(R.id.tvTimes);
        holder.txtTimer.getLayoutParams().height = height / 10;
        holder.txtName = (TextView) convertView.findViewById(R.id.tvName);
        holder.txtName.getLayoutParams().height = height / 10;
        holder.bRemove = (Button) convertView.findViewById(R.id.bRemoveTimer);
        holder.bRemove.getLayoutParams().height = height / 10;
        holder.bStartStop = (Button) convertView
                .findViewById(R.id.bStartStopTimer);
        holder.bStartStop.getLayoutParams().height = height / 10;

        convertView.setTag(holder);

    } else
        holder = (ViewHolder) convertView.getTag();

    holder.txtName.setText(rowItem.getName());
    holder.txtTimer.setText(rowItem.getTimer());

    holder.bRemove.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            final String name = holder.txtName.getText().toString();
            final String timer = holder.txtTimer.getText().toString();
            final SQLiteDatabase myDb = context.openOrCreateDatabase("timersDB", NO_SELECTION, null);
            //final int pos = position;
            AlertDialog alert;
            AlertDialog.Builder builder = new AlertDialog.Builder(context);
            builder.setTitle(R.string.delete_timer);
            builder.setMessage(context.getString(R.string.want_to_delete) + " " + name
                        + " " + context.getString(R.string.timer2));
            builder.setPositiveButton(R.string.delete,
                    new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int id) {
                            myDb.execSQL("DELETE FROM Timers WHERE "
                                    + TIME_COLUMN + " = '" + timer
                                    + "' AND " + NAME_COLUMN + " = '"
                                    + name + "';");

                            // THIS IS WHERE THE METHOD IS CALLED
                            MainActivity main = new MainActivity();
                            main.removeTimer();

                        }
                    });
                alert = builder.create();
                alert.show();
                }

这是我的日志：

02-22 17:46:15.300: E/AndroidRuntime(4508): FATAL EXCEPTION: main
02-22 17:46:15.300: E/AndroidRuntime(4508): Process: com.adrissa.kitchentimer, PID: 4508
02-22 17:46:15.300: E/AndroidRuntime(4508): java.lang.NullPointerException
02-22 17:46:15.300: E/AndroidRuntime(4508):     at com.albert.timer.MainActivity.removeTimer(MainActivity.java:115)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at com.albert.timer.CustomTimerRowAdapter$2$2.onClick(CustomTimerRowAdapter.java:272)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:167)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at android.os.Handler.dispatchMessage(Handler.java:102)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at android.os.Looper.loop(Looper.java:136)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at android.app.ActivityThread.main(ActivityThread.java:5586)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at java.lang.reflect.Method.invokeNative(Native Method)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at java.lang.reflect.Method.invoke(Method.java:515)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1268)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1084)
02-22 17:46:15.300: E/AndroidRuntime(4508):     at dalvik.system.NativeStart.main(Native Method)

【问题讨论】：

你不应该这样做MainActivity mainAct = new MainActivity();。框架处理 Activity 的创建。
我怎样才能调用该方法或从 ListViewAdapter 更新 UI？

标签： java android

【解决方案1】：

可能是因为您调用的是main.removeTimer() 而不是mainAct.removeTimer()？

【讨论】：

对不起，这只是一个类型错误，您可以在代码中看到它是正确的。

【解决方案2】：

其实就是这么简单，我只用了 5 个小时就在 Google 上找到了这个问题。

How to let OnClick in ListView's Adapter call Activity's function

【讨论】：