将活动从活动传递到广播接收器

Question

我需要将活动实例传递给停止AlarmManager的调用方法。

MainActivity

    private void setAlarm(long timeInMilis) {
    alarmManager = (AlarmManager) getSystemService((Context.ALARM_SERVICE));
    MyAlarm myAlarm = new MyAlarm(this);
    Intent intent = new Intent(this, myAlarm.getClass());
    pendingIntent = PendingIntent.getBroadcast(this, 0, intent, 0);
    alarmManager.setRepeating(AlarmManager.RTC, timeInMilis, 10000, pendingIntent);
    if (dbHelper.getProfile().getHp() >= 100) {
        stopAlarmManager();
        Toast.makeText(this, "Your character is fully healed.", Toast.LENGTH_LONG).show();
    }
}

public void stopAlarmManager() {
    if (alarmManager != null)
        alarmManager.cancel(pendingIntent);
}

和我的BroadcastReceiver

public class MyAlarm extends BroadcastReceiver {

MainActivity mainActivity;

public MyAlarm() {
}

public MyAlarm(MainActivity mainActivity) {
    this.mainActivity = mainActivity;
}

@Override
public void onReceive(Context context, Intent intent) {
    DBHelper dbHelper = new DBHelper(context);
    dbHelper.getWritableDatabase();


    dbHelper.setHp("1", dbHelper.getProfile().getHp() + 40);
    String currentHp = String.valueOf(dbHelper.getProfile().getHp());
    if (Integer.parseInt(currentHp) < dbHelper.getProfile().getHpMax())
        Toast.makeText(context, "Your character gain 40HP and now has: " + currentHp + "HP", Toast.LENGTH_LONG).show();
    else {
        Toast.makeText(context, "Your character is fully healed", Toast.LENGTH_LONG).show();
        mainActivity.stopAlarmManager();
    }
}
}

但是这段代码会导致错误：原因：java.lang.InstantiationException：空对象引用上的java.lang.Class <（...）MainActivity.stopAlarmManager>