Question

我正在制作一个简单的提醒应用。当提醒消失时，我收到空指针异常。

这里是Logcat的关键信息：引起：java.lang.NullPointerException at com.joshbgold.move.AlarmReceiver.onReceive（AlarmReceiver.java:34）

显然我没有正确初始化AlarmActivity。我知道这可能是一个简单的修复，感谢您对新的Java程序员的耐心。

AlarmReceiver.java:

import android.app.Activity;
import android.content.ComponentName;
import android.content.Context;
import android.content.Intent;
import android.media.MediaPlayer;
import android.support.v4.content.WakefulBroadcastReceiver;

/**
 * Created by JoshG on 7/6/2015.
 */

public class AlarmReceiver extends WakefulBroadcastReceiver {

    @Override
    public void onReceive(final Context context, Intent intent) {
        //MediaPlayer is used to play an mp3 file
        final MediaPlayer mediaPlayer = MediaPlayer.create(context, R.drawable.om_mani_short);

        mediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {

            @Override
            public void onCompletion(MediaPlayer mediaplayer) {
                mediaplayer.stop();
                mediaplayer.release();
            }
        });

        mediaPlayer.start();

        //this will update the UI with message
        AlarmActivity inst = AlarmActivity.instance();
        inst.setAlarmText("stretch");

        //this will send a notification message
        ComponentName comp = new ComponentName(context.getPackageName(),
                AlarmService.class.getName());
        startWakefulService(context, (intent.setComponent(comp)));
        setResultCode(Activity.RESULT_OK);
    }

}

AlarmActivity.java：

package com.joshbgold.move;

import android.app.Activity;
import android.app.AlarmManager;
import android.app.PendingIntent;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.TimePicker;
import android.widget.ToggleButton;

import java.util.Calendar;

/**
 * Created by JoshG on 7/6/2015.
 */
public class AlarmActivity extends Activity {

    AlarmManager alarmManager;
    private PendingIntent pendingIntent;
    private TimePicker alarmTimePicker;
    private static AlarmActivity inst;
    private TextView alarmTextView;

    public static AlarmActivity instance() {
        return inst;
    }

    @Override
    public void onStart() {
        super.onStart();
        inst = this;
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        alarmTimePicker = (TimePicker) findViewById(R.id.alarmTimePicker);
        alarmTextView = (TextView) findViewById(R.id.alarmText);
        ToggleButton alarmToggle = (ToggleButton) findViewById(R.id.alarmToggle);
        final Button exitButton = (Button) findViewById(R.id.exitButton);
        alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);

        final MediaPlayer mediaPlayer = MediaPlayer.create(this, R.drawable.om_mani_short);

      mediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {

            @Override
            public void onCompletion(MediaPlayer mediaplayer) {
                mediaplayer.stop();
                mediaplayer.release();
            }
        });

        mediaPlayer.start();

        View.OnClickListener quitApp = new View.OnClickListener() {  //this block stops music when exiting
            @Override
            public void onClick(View view) {

                if (mediaPlayer != null) try {
                    if (mediaPlayer.isPlaying()) {
                        mediaPlayer.stop();
                        mediaPlayer.release();
                    }
                } catch (Exception e) {
                    Log.d("Alarm Activity", e.toString());
                }

                finish();
            }
        };

        exitButton.setOnClickListener(quitApp);
    }

    public void onToggleClicked(View view) {
        if (((ToggleButton) view).isChecked()) {
            Log.d("MyActivity", "Alarm On");
            Calendar calendar = Calendar.getInstance();
            calendar.set(Calendar.HOUR_OF_DAY, alarmTimePicker.getCurrentHour());
            calendar.set(Calendar.MINUTE, alarmTimePicker.getCurrentMinute());
            Intent myIntent = new Intent(AlarmActivity.this, AlarmReceiver.class);
            pendingIntent = PendingIntent.getBroadcast(AlarmActivity.this, 0, myIntent, 0);
            alarmManager.set(AlarmManager.RTC, calendar.getTimeInMillis(), pendingIntent);
        } else {
            alarmManager.cancel(pendingIntent);
            setAlarmText("Alarm Off");
            Log.d("MyActivity", "Alarm Off");
        }
    }

    public void setAlarmText(String alarmText) {
        alarmTextView.setText(alarmText);
    }
}

警报Service.java

package com.joshbgold.move;

import android.app.IntentService;
import android.app.NotificationManager;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.support.v4.app.NotificationCompat;
import android.util.Log;

/**
 * Created by JoshG on 7/6/2015.
 */
public class AlarmService extends IntentService {
    private NotificationManager alarmNotificationManager;

    public AlarmService() {
        super("AlarmService");
    }

    @Override
    public void onHandleIntent(Intent intent) {
        sendNotification("stretch");
    }

    private void sendNotification(String msg) {
        Log.d("AlarmService", "Preparing to send notification...: " + msg);
        alarmNotificationManager = (NotificationManager) this
                .getSystemService(Context.NOTIFICATION_SERVICE);

        PendingIntent contentIntent = PendingIntent.getActivity(this, 0,
                new Intent(this, AlarmActivity.class), 0);

        NotificationCompat.Builder alarmNotificationBuilder = new NotificationCompat.Builder(
                this).setContentTitle("Reminder").setSmallIcon(R.mipmap.ic_launcher)
                .setStyle(new NotificationCompat.BigTextStyle().bigText(msg))
                .setContentText(msg);


        alarmNotificationBuilder.setContentIntent(contentIntent);
        alarmNotificationManager.notify(1, alarmNotificationBuilder.build());
        Log.d("AlarmService", "Notification sent.");
    }
}

这是Logcat的内容。我把它设置为Log Level of Error：

07-07 18:53:13.119  13065-13065/com.joshbgold.move E/﹕ mali: REVISION=Linux-r3p2-01rel2 BUILD_DATE=Mon Nov 18 21:41:36 KST 2013
07-07 19:00:11.569  16746-16746/com.joshbgold.move E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start receiver com.joshbgold.move.AlarmReceiver: java.lang.NullPointerException
            at android.app.ActivityThread.handleReceiver(ActivityThread.java:2469)
            at android.app.ActivityThread.access$1600(ActivityThread.java:158)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1372)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:176)
            at android.app.ActivityThread.main(ActivityThread.java:5365)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.joshbgold.move.AlarmReceiver.onReceive(AlarmReceiver.java:34)
            at android.app.ActivityThread.handleReceiver(ActivityThread.java:2462)
            at android.app.ActivityThread.access$1600(ActivityThread.java:158)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1372)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:176)
            at android.app.ActivityThread.main(ActivityThread.java:5365)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

//this will update the UI with message
AlarmActivity inst = AlarmActivity.instance();
inst.setAlarmText("stretch");

inst是null。您应该检查AlarmActivity.instance()

public static AlarmActivity instance() {
    if (inst == null) {
        // Do something such as inst = ...
    }

    return inst;
}

Answer 2

如果这是班级，第34行是

 inst.setAlarmText("stretch");

指向上面的行没有获得实际活动

AlarmActivity inst = AlarmActivity.instance();

检查AlarmActivity类，确保在使用instance（）方法创建inst时，inst不为null。

您可以发布AlarmActivity类以了解如何创建/获取实例吗？

Answer 3

我右转了一圈，并在提醒/闹钟响起时执行了不同的实施。虽然我正在做一些与最初预期不同的事情，但上面给出的答案让我朝着正确的方向前进。

以下是我用于AlarmReceiver.java的内容：

    package com.joshbgold.move;    

    import android.content.Context;
    import android.content.Intent;
    import android.support.v4.content.WakefulBroadcastReceiver;

    public class AlarmReceiver extends WakefulBroadcastReceiver {

        @Override
        public void onReceive(final Context context, Intent intent) {
            //this will change to new activity for Reminder(s) at the appropriate time
            Intent myIntent = new Intent();
            myIntent.setClassName("com.joshbgold.move", "com.joshbgold.move.ReminderActivity");
            myIntent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            context.startActivity(myIntent);
     }
    }

空指针异常 - 我没有初始化什么？

3 个答案: