Android：NullPointerException 创建新的意图

Question

I always get a Nullpointer Exception when creating a new Intent Object. My intention is to create a new Activity which opens a new View with its "onCreate" method.

This is my Main Activity:

public class MainActivity extends Activity {
    public void activateTimer(){
        // some code ....           
        MyTimer timer = new MyTimer(MainActivity.this);
    }
}

This is my Timer Class:

public class MyTimer extends MainActivity{
    private Context contextMain;
    public MyTimer(Context context){
        this.contextMain = context;
        // some more code ...
        openMyActivity(contextMain); 
}
public void openMyActivity(Context contextMain){
    // some code ...
    // The next line throws the NPE!
Intent intent = new Intent(contextMain, MyNotificationActivity.class);
this.startActivity(intent);
    }
}

My Activity which should be created:

public class MyNotificationActivity extends Activity {  
    @Override
    public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.notification);
 } 
}

Here is the error log:

03-21 12:47:51.949: W/dalvikvm(2442): threadid=11: thread exiting with uncaught exception (group=0x41465700)
03-21 12:47:51.959: E/AndroidRuntime(2442): FATAL EXCEPTION: Timer-0
03-21 12:47:51.959: E/AndroidRuntime(2442): java.lang.NullPointerException
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivityForResult(Activity.java:3390)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivityForResult(Activity.java:3351)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivity(Activity.java:3587)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at android.app.Activity.startActivity(Activity.java:3555)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at com.myApp.MyTimer.openNotificationInMain(MyTimer.java:135)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at com.myApp.MyTimer.prompt(MyTimer.java:80)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at com.myApp.MyTimer$1.run(MyTimer.java:38)
03-21 12:47:51.959: E/AndroidRuntime(2442):     at java.util.Timer$TimerImpl.run(Timer.java:284)

This is my Manifest.xml File:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.myApp"
    android:versionCode="1"
    android:versionName="1.0" >
<uses-sdk
    android:minSdkVersion="8"
    android:targetSdkVersion="18" />

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.myApp.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    
    <activity android:name="com.myApp.MyNotifikationActivity"
              android:label="@string/app_name"
              android:parentActivityName="com.myApp.MainActivity"  >
        <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
       </intent-filter>
        <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="com.myApp.MainActivity" />            
    </activity>         
</application>
</manifest>

Answer 1

Your MyTimer extends Activity but here you're instantiating it yourself:

MyTimer timer = new MyTimer(MainActivity.this)

Now, when you do this, the activity isn't properly initialized to be used as an activity or a context and all kinds of failures are possible.

Seems like you should remove the extends Activity from MyTimer altogether and pass in an Activity or Context argument to those methods that need it. For example,

public void openMyActivity(Activity act) {
  // ...
  act.startActivityForResult(...);

Answer 2

Why do you extend Mytimer to MainActivity?. I don't see the need of that

change it to ....

public class MyTimer{
private Context contextMain;
   public MyTimer(Context context){
    this.contextMain = context;
    // some more code ...
    openMyActivity(contextMain); 
  }
 public void openMyActivity(Context contextMain){
// some code ...
// The next line throws the NPE!
Intent intent = new Intent(contextMain, MyNotificationActivity.class);
this.startActivity(intent);
}

}