Android闹钟NullException

Question

我是Android Studio的初学者。

目前，我试图通过使用Android Studio制作闹钟。试图运行代码时出现错误。当它开始运行时，它会给我这些错误。

E/AndroidRuntime: FATAL EXCEPTION: main 
Process: com.google.alarmclock, PID: 2837 
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.google.alarmclock/com.google.alarmclock.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2548) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6077) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference 
    at android.content.ComponentName.<init>(ComponentName.java:128) 
    at android.content.Intent.<init>(Intent.java:4868) 
    at com.google.alarmclock.MainActivity.<init>(MainActivity.java:30) 
    at java.lang.Class.newInstance(Native Method) 
    at android.app.Instrumentation.newActivity(Instrumentation.java:1078) 
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2538) 
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)  
    at android.app.ActivityThread.-wrap12(ActivityThread.java)  
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)  
    at android.os.Handler.dispatchMessage(Handler.java:102)  
    at android.os.Looper.loop(Looper.java:154)  
    at android.app.ActivityThread.main(ActivityThread.java:6077)  
    at java.lang.reflect.Method.invoke(Native Method)  
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)  
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)  

这是我的java文件。

Alarm_Receiver.java

package com.google.alarmclock; 

import android.content.BroadcastReceiver; 
import android.content.Context; 
import android.content.Intent; 
import android.util.Log; 

public class Alarm_Receiver extends BroadcastReceiver { 
    @Override 
    public void onReceive(Context context, Intent intent) { 
     Log.e("We are in the Receiver","yaa"); 

     //fetch extra strings from the intent 
     String get_your_string = intent.getExtras().getString("extra"); 

     Log.e("What is the key?", get_your_string); 

     //create an intent to the ringtone service 
     Intent service_intent=new Intent(context,RingtonePlayingService.class); 

     //pass the extra string from Main Activity to the Ringtone Playing Service 
     service_intent.putExtra("extra",get_your_string); 

     //start the ringtone service 
     context.startService(service_intent); 
    } 
} 

RingtonePlayingService.java

package com.google.alarmclock; 

import android.app.Notification; 
import android.app.NotificationManager; 
import android.app.PendingIntent; 
import android.app.Service; 
import android.content.Intent; 
import android.media.MediaPlayer; 
import android.os.IBinder; 
import android.support.annotation.Nullable; 
import android.util.Log; 
import android.widget.Toast; 

import java.security.Provider; 
import java.util.List; 
import java.util.Map; 

public class RingtonePlayingService extends Service { 

    MediaPlayer media_song; 
    int startId; 
    boolean isRunning; 

    @Nullable 
    @Override 
    public IBinder onBind(Intent intent) { 
     return null; 
    } 

    @Override 
    public int onStartCommand(Intent intent, int flags, int startId) { 
     Log.i("LocalService", "Received start id " + startId + ": " + intent); 

     //fetch the extra string values 
     String state = intent.getExtras().getString("extra"); 

     Log.e("Ringtone extra is",state); 

     //this converts the extra strings from the intent 
     //to start IDs, values 0 or 1 
     assert state !=null; 
     switch (state) { 
      case "alarm on": 
       startId = 1; 
       break; 
      case "alarm off": 
       startId = 0; 
       Log.e("Start ID is ",state); 
       break; 
      default: 
       startId = 0; 
       break; 
     } 

     //if else statements 

     //if there is no music playing, and the user pressed "alarm on" 
     //music should start playing 
     if (!this.isRunning && startId==1) { 
      Log.e("there is no music, ","and you want start"); 
      //create an instance of the media player 
      media_song = MediaPlayer.create(this,R.raw.i_am_singer); 
      media_song.start(); 

      this.isRunning=true; 
      this.startId=0; 

      //put the notification here,test it out 
      //notification 
      //set up the notification service 
      NotificationManager notify_manager=(NotificationManager) 
        getSystemService(NOTIFICATION_SERVICE); 
      //set up an intent that goes to the Main Activity 
      Intent intent_main_activity=new Intent(this.getApplicationContext(),MainActivity.class); 

      //set up a pending intent 
      PendingIntent pending_intent_main_activity=PendingIntent.getActivity(this,0, 
        intent_main_activity,0); 

      //make the notification parameters 
      Notification notification_popup=new Notification.Builder(this) 
        .setContentTitle("An alarm is going off!") 
        .setContentText("Click Me") 
        .setContentIntent(pending_intent_main_activity) 
        .setAutoCancel(true) 
        .build(); 

      //set up the notification call command 
      notify_manager.notify(0,notification_popup); 

     } 
     //if there is music playing, and the user pressed "alarm off" 
     //music should stop playing 
     else if (this.isRunning && startId == 0) { 
      Log.e("there is music, ","and you want end"); 

      //stop the ringtone 
      media_song.stop(); 
      media_song.reset(); 

      this.isRunning=false; 
      this.startId=0; 

     } 
     //there are if user presses random buttons 
     //just to bug-proof the app 
     //if there is no music playing, and the user pressed "alarm off" 
     //do nothing 
     else if (!this.isRunning && startId == 0) { 
      Log.e("there is no music, ","and you want end"); 

      //stop the ringtone 
      this.isRunning=false; 
      this.startId=0; 

     } 
     //if there is music playing and the user pressed "alarm on" 
     //do nothing 
     else if (this.isRunning && startId ==1) { 
      Log.e("there is music, ","and you want start"); 

      this.isRunning =true; 
      this.startId=1; 

     } 
     //can't think of anything else, just to catch the odd event 
     else { 
      Log.e("else ","somehow you reached "); 
     } 
     return START_NOT_STICKY; 
    } 

    @Override 
    public void onDestroy() { 
     // Tell the user we stopped. 
     Log.e("on Destroy called","ta da"); 
     super.onDestroy();; 
     this.isRunning=false; 
     Toast.makeText(this, "on Destroy called ", Toast.LENGTH_SHORT).show(); 
    } 
} 

MainActivity.java

package com.google.alarmclock; 

import android.annotation.TargetApi; 
import android.app.AlarmManager; 
import android.app.PendingIntent; 
import android.content.Context; 
import android.content.Intent; 
import android.os.Build; 
import android.os.Bundle; 
import android.support.design.widget.FloatingActionButton; 
import android.support.design.widget.Snackbar; 
import android.support.v7.app.AppCompatActivity; 
import android.support.v7.widget.Toolbar; 
import android.view.View; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.widget.Button; 
import android.widget.TextView; 
import android.widget.TimePicker; 
import java.util.Calendar; 

public class MainActivity extends AppCompatActivity { 
    //to make alarm manager 
    AlarmManager alarm_manager; 
    TimePicker alarm_timepicker; 
    TextView update_text; 
    Context context; 
    PendingIntent pendingIntent; 
    final Intent my_intent=new Intent(this.context,Alarm_Receiver.class); 

    //create an instance of a calender 
    final Calendar calendar= Calendar.getInstance(); 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
     setSupportActionBar(toolbar); 
     this.context = this; 

     //initialize alarm manager 
     alarm_manager=(AlarmManager)getSystemService(ALARM_SERVICE); 

     //initialize timepicker 
     alarm_timepicker=(TimePicker)findViewById(R.id.timePicker); 

     //initialize our text update box 
     update_text=(TextView)findViewById(R.id.update_text); 



     //initialize start buttons 
     // Button start_alarm=(Button)findViewById(R.id.start_alarm); 



     //initialize the stop button 
     //Button stop_button=(Button)findViewById(R.id.end_alarm); 

     //Create an intent to the Alarm Receiver class 


    } 

    //create an onClick listener to start the alarm 
    @TargetApi(Build.VERSION_CODES.M) 
    public void onClickStartAlarm(View view) 
    { 
     //setting calendar instance with the hour and minute that have picked 
     //on the time picker 
     calendar.set(Calendar.HOUR_OF_DAY,alarm_timepicker.getHour()); 
     calendar.set(Calendar.MINUTE,alarm_timepicker.getMinute()); 

     //get the string values of the hour and minute 
     int hour=alarm_timepicker.getHour(); 
     int minute=alarm_timepicker.getMinute(); 

     //convert the int values to Strings 
     String hour_string=String.valueOf(hour); 
     String minute_string=String.valueOf(minute); 

     //convert 24 hour time to 12 hour time 
     if(hour>12) 
     { 
      hour_string="0" + String.valueOf(hour -12); 
     } 


     if(minute<10) 
     { 
      //10:7 --> 10:07 
      minute_string=String.valueOf(minute); 
     } 

     //method that changes the update text Textbox 
     set_alarm_text("Alarm On: " + hour_string + ":" +minute_string); 

     //put in extra string into my_intent 
     //tells the clock that you pressed the "alarm on" button 
     my_intent.putExtra("extra","on"); 


     //Create a pending intent that delays the intent 
     //until the specified calendar time 
     pendingIntent = PendingIntent.getBroadcast(MainActivity.this,0,my_intent,PendingIntent.FLAG_UPDATE_CURRENT); 

     //set the alarm manager 
     alarm_manager.set(AlarmManager.RTC_WAKEUP,calendar.getTimeInMillis(),pendingIntent); 
    } 

    private void set_alarm_text(String output) { 
     update_text.setText(output); 
    } 

    //create an onClick listener to stop the alarm or undo an alarm set 
    public void onClickStopAlarm(View view) 
    { 
     //method that changes the update text Textbox 
     set_alarm_text("Alarm off!"); 

     //Cancel the alarm 
     alarm_manager.cancel(pendingIntent); 

     //put extra string into my_intent 
     //tells the clock that you pressed the "alarm off" button 
     my_intent.putExtra("extra","off"); 


     //Stop the ringtone 
     sendBroadcast(my_intent); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_settings) { 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 
} 

的AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?> 

<application 
    android:allowBackup="true" 
    android:icon="@mipmap/ic_launcher" 
    android:label="@string/app_name" 
    android:supportsRtl="true" 
    android:theme="@style/AppTheme"> 
    <activity 
     android:name=".MainActivity" 
     android:label="@string/app_name" 
     android:theme="@style/AppTheme.NoActionBar"> 
     <intent-filter> 
      <action android:name="android.intent.action.MAIN" /> 

      <category android:name="android.intent.category.LAUNCHER" /> 
     </intent-filter> 
    </activity> 

    <receiver android:name=".Alarm_Receiver"/> 
    <service android:name=".RingtonePlayingService" 
     android:enabled="true"/> 
</application> 

的build.gradle

apply plugin: 'com.android.application' 

android { 
    compileSdkVersion 24 
    buildToolsVersion "24.0.2" 
    defaultConfig { 
     applicationId "com.google.alarmclock" 
     minSdkVersion 16 
     targetSdkVersion 24 
     versionCode 1 
     versionName "1.0" 
     testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner" 
    } 
    buildTypes { 
     release { 
      minifyEnabled false 
      proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro' 
     } 
    } 
} 

dependencies { 
    compile fileTree(dir: 'libs', include: ['*.jar']) 
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', { 
     exclude group: 'com.android.support', module: 'support-annotations' 
    }) 
    compile 'com.android.support:appcompat-v7:24.2.1' 
    compile 'com.android.support:design:24.2.1' 
    testCompile 'junit:junit:4.12' 
} 

请帮我解决问题。谢谢

Answer 1

你应该申报Calender对象了侧onCreate并初始化它里面onCreate。

Calendar calendar; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    calendar= Calendar.getInstance(); 
... 
} 

另外，你仍然会得到NullPointerException，因为在这条线

final Intent my_intent=new Intent(this.context,Alarm_Receiver.class); 

的情况下将是无效的。所以你可以做同样的事情。

Intent my_intent; 
Calendar calendar; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    this.context = this; 

    calendar= Calendar.getInstance(); 

    my_intent=new Intent(this.context,Alarm_Receiver.class); 

... 
} 

快乐编码。

Answer 2

下面这条线final Calendar calendar = Calendar.getInstance();如下。

初始化它onCreate()

...  
//create an instance of a calender 
//final Calendar calendar= Calendar.getInstance(); 
Calendar calendar; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    calendar= Calendar.getInstance(); 
... 
}