Question

I’m debugging my application, and getting a ‘strange’ exception. It’s not consistent and therefore really hard to solve. I’ll post some code below.

The class that gives my exception:

private ArrayList<String> idList;

@Override
public void onCreate( Bundle savedInstanceState )
{
    super.onCreate( savedInstanceState );
    if ( idList == null )
    {
        idList = new ArrayList<String>();
    }
}

@Override
public void finishFromChild( Activity child )
{
    LocalActivityManager manager = getLocalActivityManager();
    int index = idList.size()-1;

    if ( index < 1 )
    {
        finish();
        return;
    }

    manager.destroyActivity( idList.get( index ), true );
    idList.remove( index ); index--;
    String lastId = idList.get( index );
    Activity lastActivity = manager.getActivity( lastId );
    Intent lastIntent = lastActivity.getIntent();
    Window newWindow = manager.startActivity( lastId, lastIntent );
    setContentView( newWindow.getDecorView() );
}

public void startChildActivity( String id, Intent intent )
{
    if ( "restart".equalsIgnoreCase( id ) )
    {
        idList.clear();
    }

    Window window = getLocalActivityManager().startActivity( id, intent.addFlags( Intent.FLAG_ACTIVITY_CLEAR_TOP ) );
    if ( window != null )
    {
        idList.add( id );
        setContentView( window.getDecorView() );
    }
}

public Activity getCurrentActivity()
{
    int length = idList.size();
    if ( idList.isEmpty() ) {
        return null;
    }
    else
    {
        return getLocalActivityManager().getActivity( idList.get( length-1 ) );
    }
}

@Override
public boolean onKeyDown( int keyCode, KeyEvent event )
{
    if ( keyCode == KeyEvent.KEYCODE_BACK )
    {
        return true;
    }
    return super.onKeyDown( keyCode, event );
}

@Override
public boolean onKeyUp( int keyCode, KeyEvent event )
{
    if ( keyCode == KeyEvent.KEYCODE_BACK )
    {
        onBackPressed();
        return true;
    }
    return super.onKeyUp( keyCode, event );
}

@Override
public void onBackPressed()
{
    int length = idList.size();
    if ( length > 1 )
    {
        Activity current = getLocalActivityManager().getActivity( idList.get( length-1 ) );
        current.finish();
    }
}

The exception itself:

09-30 08:44:12.571: ERROR/AndroidRuntime(10557): FATAL EXCEPTION: main
09-30 08:44:12.571: ERROR/AndroidRuntime(10557): java.lang.NullPointerException
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.dezide.android.troubleshooter.view.TabGroupActivity.finishFromChild(TabGroupActivity.java:46)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.app.Activity.finish(Activity.java:3290)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.dezide.android.troubleshooter.view.TabGroupActivity.onBackPressed(TabGroupActivity.java:106)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.dezide.android.troubleshooter.view.TabGroupActivity.onKeyUp(TabGroupActivity.java:93)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.KeyEvent.dispatch(KeyEvent.java:1281)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.app.Activity.dispatchKeyEvent(Activity.java:2075)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1673)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:796)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:796)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:796)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.widget.TabHost.dispatchKeyEvent(TabHost.java:275)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:796)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:796)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchKeyEvent(PhoneWindow.java:1697)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.android.internal.policy.impl.PhoneWindow.superDispatchKeyEvent(PhoneWindow.java:1111)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.app.Activity.dispatchKeyEvent(Activity.java:2070)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1673)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewRoot.deliverKeyEventToViewHierarchy(ViewRoot.java:2493)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewRoot.handleFinishedEvent(ViewRoot.java:2463)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1752)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.os.Looper.loop(Looper.java:144)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at android.app.ActivityThread.main(ActivityThread.java:4937)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at java.lang.reflect.Method.invokeNative(Native Method)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at java.lang.reflect.Method.invoke(Method.java:521)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
09-30 08:44:12.571: ERROR/AndroidRuntime(10557):     at dalvik.system.NativeStart.main(Native Method)

The problem is that it does not always fail at the same place. Sometimes I can press back 20 times before the exception occurs, sometimes only 5.

EDIT: Adding more code.

My GuideActivity, which is where you make the steps, and launches each of them as a new activity:

public void onCreate( Bundle savedInstanceState )
{
    super.onCreate( savedInstanceState );

    if ( "startguide".equalsIgnoreCase( getIntent().getStringExtra( "action" ) ) )
    {
        try
        {
            startGuide( getIntent().getStringExtra( "guide" ) );
        }
        catch( Exception e )
        {
            e.printStackTrace();
        }
    }
    else if ( "nextstep".equalsIgnoreCase( getIntent().getStringExtra( "action" ) ) )
    {
        String session = getIntent().getStringExtra( "session" );
        String step = getIntent().getStringExtra( "step" );
        String response = getIntent().getStringExtra( "response" );

        try
        {
            nextGuideStep( session, step, response );
        }
        catch( Exception e )
        {
            e.printStackTrace();
        }
    }
}

public GuideStep getCurrentStep()
{
    return currentStep;
}

public void setCurrentStep( GuideStep currentStep )
{
    this.currentStep = currentStep;
}

public void startGuide( String name ) throws Exception
{
    GuideStep guideStep = model.startGuide( name );
    currentStep = guideStep;

    setContentView ( R.layout.main );
    LinearLayout linearLayout = ( LinearLayout ) findViewById( R.id.linearLayout );
    TextView stepTitle = ( TextView ) findViewById( R.id.stepTitle );
    TextView explanation = ( TextView ) findViewById( R.id.explanation );

    stepTitle.setText( guideStep.getStepTitle() );

    if ( guideStep.getExplanation() != "" )
    {
        explanation.setText( Html.fromHtml( guideStep.getExplanation() ) );
    }

    responses = guideStep.getResponses();

    ListView listView = new ListView( this );
    linearLayout.addView( listView );
    listView.setAdapter( new CustomAdapter( this, R.layout.list_item, responses ) );
    listView.setTextFilterEnabled( true );
    listView.setOnItemClickListener( new OnItemClickListener()
    {
        public void onItemClick( AdapterView<?> parent, View view,
            int position, long id ) {
            try
            {
                Intent i = new Intent().setClass( getParent(), GuideActivity.class );
                i.putExtra( "action", "nextstep" );
                i.putExtra( "session", currentStep.getSession() );
                i.putExtra( "step", currentStep.getStep() );
                i.putExtra( "response", currentStep.getResponse( position ).getId() );
                TabGroupActivity parentActivity = ( TabGroupActivity )getParent();
                parentActivity.startChildActivity( currentStep.getStepTitle(), i );
            }
            catch( Exception e )
            {
                e.printStackTrace();
            }
        }
    });
}

public void nextGuideStep( String session, String step, String responseId ) throws Exception
{
    GuideStep guideStep = model.nextGuideStep( session, step, responseId );
    currentStep = guideStep;

    setContentView ( R.layout.main );
    LinearLayout linearLayout = ( LinearLayout ) findViewById( R.id.linearLayout );
    TextView stepTitle = ( TextView ) findViewById( R.id.stepTitle );
    TextView explanation = ( TextView ) findViewById( R.id.explanation );
    TextView didThisAnswerYourQuestion = ( TextView ) findViewById( R.id.didThisAnswerYourQuestion );
    ScrollView scrollView = ( ScrollView ) findViewById( R.id.scrollView );

    stepTitle.setText( guideStep.getStepTitle() );

    if ( guideStep.getExplanation() != "" )
    {
        explanation.setText( Html.fromHtml( guideStep.getExplanation() ) );
    }
    responses = guideStep.getResponses();

    if ( guideStep.getStepTitle() != "" )
    {
        ListView listView = new ListView( this );
        linearLayout.addView( listView );
        listView.setAdapter( new CustomAdapter( this, R.layout.list_item, responses ) );
        listView.setTextFilterEnabled( true );
        listView.setOnItemClickListener( new OnItemClickListener() {
            public void onItemClick( AdapterView<?> parent, View view,
                int position, long id ) {
                try
                {
                    Intent i = new Intent().setClass( getParent(), GuideActivity.class );
                    i.putExtra( "action", "nextstep" );
                    i.putExtra( "session", currentStep.getSession() );
                    i.putExtra( "step", currentStep.getStep() );
                    i.putExtra( "response", currentStep.getResponse( position ).getId() );
                    TabGroupActivity parentActivity = ( TabGroupActivity )getParent();
                    parentActivity.startChildActivity( currentStep.getStepTitle(), i );
                }
                catch( Exception e )
                {
                    e.printStackTrace();
                }
            }
          });
    }
    else {
        stepTitle.setVisibility( View.GONE );
        didThisAnswerYourQuestion.setVisibility( View.GONE );
        RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams( RelativeLayout.LayoutParams.FILL_PARENT, RelativeLayout.LayoutParams.FILL_PARENT );
        params.addRule( RelativeLayout.ALIGN_PARENT_TOP, RelativeLayout.TRUE );
        params.height = 350;
        scrollView.setLayoutParams( params );
    }
}

public boolean availableForFeedback()
{
    return currentStep != null;
}

}

My starting Activity:

public void onCreate( Bundle savedInstanceState )
{
    super.onCreate( savedInstanceState );

    try
    {
        populateSections();
    }
    catch( Exception e )
    {
        e.printStackTrace();
    }
}

private void populateSections() throws Exception
{
    setContentView( R.layout.sections );

    ArrayList<XmlSection> sections = new ArrayList<XmlSection>();

    Portal portal = model.getPortal();
    sections = portal.getSections();

    final ArrayList<XmlSection> tempSections = sections;

    ListView listView = ( ListView ) findViewById( R.id.sectionList );

    listView.setAdapter( new CustomAdapter( this, R.layout.list_item, sections ) );
    listView.setTextFilterEnabled( true );
    listView.setOnItemClickListener( new OnItemClickListener()
    {
        public void onItemClick( AdapterView<?> parent, View view,
            int position, long id )
        {
            try
            {
                Intent i = new Intent().setClass( SectionActivity.this, JAndroidTroubleshooterActivity.class );
                i.putExtra( "name", tempSections.get( position ).getName() );
                startActivity( i );
            }
            catch( Exception e )
            {
                e.printStackTrace();
            }
        }
    });
}

The activity that my main activity launches:

@Override
public void onCreate( Bundle savedInstanceState )
{
    super.onCreate( savedInstanceState );

    Resources res = getResources();
    TabHost tabHost = getTabHost();
    TabHost.TabSpec spec;
    Intent intent;

    intent = new Intent().setClass( this, TabGroupGuideActivity.class );
    intent.putExtra( "name", getIntent().getStringExtra( "name" ) );
    spec = tabHost.newTabSpec( "guides" ).setIndicator( "Guides",
                      res.getDrawable( R.drawable.guides ) )
                  .setContent( intent );
    tabHost.addTab( spec );

    intent = new Intent().setClass( this, TabGroupFaqActivity.class );
    intent.putExtra( "name", getIntent().getStringExtra( "name" ) );
    spec = tabHost.newTabSpec( "faqs" ).setIndicator( "FAQs",
                      res.getDrawable( R.drawable.faq ) )
                  .setContent( intent );
    tabHost.addTab( spec );
}

private Activity getCurrentTab()
{
    Activity activity = getLocalActivityManager().getActivity( getTabHost().getCurrentTabTag() );
    return activity;
}

private void optionRestart()
{
    if ( getCurrentTab() instanceof TabGroupFaqActivity )
    {
        Intent i = new Intent().setClass( this, FaqActivity.class );
        TabGroupFaqActivity activity = ( TabGroupFaqActivity ) getCurrentActivity();
        i.putExtra( "name", activity.getCurrentSection() );
        activity.startChildActivity( "restart", i );
    }
    if ( getCurrentTab() instanceof TabGroupGuideActivity )
    {
        Intent i = new Intent().setClass( this, GuideListActivity.class );
        TabGroupGuideActivity activity = ( TabGroupGuideActivity ) getCurrentActivity();
        i.putExtra( "name", activity.getCurrentSection() );
        activity.startChildActivity( "restart", i );
    }
}

private void optionFeedback()
{
    if ( getCurrentTab() instanceof TabGroupFaqActivity )
    {
        Activity currentTab = ( ( TabGroupFaqActivity ) getCurrentTab() ).getCurrentActivity();
        Article currentArticle = ( ( FaqActivity ) currentTab ).getCurrentArticle();

        if ( currentArticle != null )
        {
            Intent i = new Intent().setClass( this, FeedbackActivity.class );
            i.putExtra( "perform", "feedbackfaq" );
            i.putExtra( "title", currentArticle.getTitle() );
            startActivity( i );
        }
        else
        {
            Toast toast = Toast.makeText( getApplicationContext(), "You have to select an article to submit feedback", 500 );
            toast.show();
        }
    }
    if ( getCurrentTab() instanceof TabGroupGuideActivity )
    {
        Activity currentTab = ( ( TabGroupGuideActivity ) getCurrentTab() ).getCurrentActivity();
        GuideStep currentStep = ( ( GuideActivity ) currentTab ).getCurrentStep();

        if ( currentStep != null && "false".equalsIgnoreCase( currentStep.getTerminalStep() ) )
        {
            Intent i = new Intent().setClass( this, FeedbackActivity.class );
            i.putExtra( "perform", "feedbackguide" );
            i.putExtra( "session", currentStep.getSession() );
            i.putExtra( "action", currentStep.getStepTitle() );
            i.putExtra( "title", currentStep.getGuideTitle() );
            startActivity( i );
        }
        else
        {
            Toast toast = Toast.makeText( getApplicationContext(), "You cannot submit feedback for the last step of a guide", 500 );
            toast.show();
        }
    }
}

private void optionHome()
{
    Intent i = new Intent().setClass( this, SectionActivity.class );
    i.setFlags( Intent.FLAG_ACTIVITY_CLEAR_TOP );
    startActivity( i );
}

@Override
public void onConfigurationChanged( Configuration newConfig )
{
    super.onConfigurationChanged( newConfig );
}

@Override
public boolean onCreateOptionsMenu( Menu menu )
{
    MenuInflater inflater = getMenuInflater();
    inflater.inflate( R.menu.menu, menu );
    return true;
}

@Override
public boolean onOptionsItemSelected( MenuItem item )
{
    switch ( item.getItemId() )
    {
    case R.id.menuRestart:
        optionRestart();
        return true;
    case R.id.menuHome:
        optionHome();
        return true;
    case R.id.menuFeedback:
        optionFeedback();
        return true;
    default:
        return super.onOptionsItemSelected( item );
    }
}

My tab group activity:

@Override
public void onCreate( Bundle savedInstanceState )
{
    super.onCreate( savedInstanceState );

    currentSection = getIntent().getStringExtra( "name" );

    startChildActivity( "GuideListActivity",
         new Intent( this, GuideListActivity.class ).putExtra( "name", getIntent().getStringExtra( "name" ) ) );
}

public String getCurrentSection()
{
    return currentSection;
}

And last, my activity that list guides:

public void onCreate( Bundle savedInstanceState )
{
super.onCreate( savedInstanceState );

    try
    {
        populateGuideList( getIntent().getStringExtra( "name" ) );
    }
    catch( Exception e )
    {
        e.printStackTrace();
    }
}

public void populateGuideList( String name ) throws Exception
{
    setContentView( R.layout.guides );

    Section section = model.getSection( name );
    guides = section.getGuides();
    ListView listView = ( ListView ) findViewById( R.id.guideList );

    listView.setAdapter( new CustomAdapter( this, R.layout.list_item, guides ) );
    listView.setTextFilterEnabled( true );
    listView.setOnItemClickListener( new OnItemClickListener()
    {
        public void onItemClick( AdapterView<?> parent, View view,
            int position, long id )
        {
            try
            {
                XmlGuide guide = guides.get( position );
                Intent i = new Intent().setClass( getParent(), GuideActivity.class );
                i.putExtra( "guide", guide.getName() );
                i.putExtra( "action", "startguide" );
                TabGroupActivity parentActivity = ( TabGroupActivity )getParent();
                parentActivity.startChildActivity( guide.getName(), i );
            }
            catch( Exception e )
            {
                e.printStackTrace();
            }
        }
    });
}

Answer 1

I think it may help:

public void startChildActivity(String id, Intent intent) { 
    id+=System.currentTimeMillis();
    //rest of your code.
}

Edit: it need a little elaboration.

There are Activity A, B and C

• You start A with Id "A"
• then you start B with Id "B"
• then you start C with Id "C"
• and from C you again start another instance of B with id "B" (it may
  happen, because B may be a ResultListScreen which may called for
  several time with Load More type button)

The problem is that in ArrayList<String> idList when you finish from last B

manager.destroyActivity( idList.get( index ), true );

line is called which destroys Activity having id "B", which are 2 in stack.
both B are deleted and C is pushed. and when you finish C and there is no B in the stack but its Id is available is ArrayList<String> idList and you get its Activity which is null if not found.

That’s all.

id+=System.currentTimeMillis();

in this line of code you assign a unique id every time you start new Activity by using System’s current time.