Android 返回活动时出现NullPointerException

我正在调试我的应用程序，并得到一个“奇怪”的异常。这是不一致的，因此很难解决。我将在下面发布一些代码

给出我的异常的类：

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();
    }
}

问题是它并不总是在同一个地方失败。有时我可以在异常发生前按20次，有时只有5次

编辑：添加更多代码

My GuideActivity（我的指南活动），您可以在其中制定步骤，并将每个步骤作为新活动启动：

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;
}

@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;
}

我的选项卡组活动：

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;
}

@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;
}

最后，我的活动列表指导：

创建时的公共void（Bundle savedInstanceState） { super.onCreate（savedInstanceState）

试试看
{
populateGuideList（getIntent（）.getStringExtra（“名称”）；
}
捕获（例外e）
{
e、 printStackTrace（）；
}
}
public void populateGuideList（字符串名称）引发异常
{
setContentView（R.layout.guides）；
Section=model.getSection（名称）；
guides=section.getGuides（）；
ListView ListView=（ListView）findViewById（R.id.guideList）；
setAdapter（新的CustomAdapter（this，R.layout.list_项，guides））；
setTextFilterEnabled（true）；
setOnItemClickListener（新的OnItemClickListener（）
{
public void onItemClick（AdapterView父级、视图、，
内部位置，长id）
{
尝试
{
XmlGuide=guides.get（位置）；
Intent i=new Intent（）.setClass（getParent（），GuideActivity.class）；
i、 putExtra（“guide”，guide.getName（））；
i、 putExtra（“行动”、“开始指南”）；
TabGroupActivity parentActivity=（TabGroupActivity）getParent（）；
parentActivity.startChildActivity（guide.getName（），i）；
}
捕获（例外e）
{
e、 printStackTrace（）；
}
}
});
}

NullPointerException主要发生在您试图指向不存在的内容时……例如：您忘记在资源文件夹中添加图像，并说是setBackground（图像）或者您忘了在清单文件中提到某个活动，正试图使用Intent访问该活动。请确保您的res文件夹中有所有资源，并且清单文件中提到的所有活动都在，我认为这可能会有所帮助：

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

编辑：它需要一点细化

有

活动

A

、

B

和

C

• 您可以使用Id

  “A”启动
  A

• 然后用Id

  “B”启动
  B

• 然后用Id

  “C”启动
  C

• 然后从C再次启动另一个id为

  “B”

  的

  B

  实例（它可能 发生，因为

  B

  可能是一个可能需要 多次使用“加载更多类型”按钮）

问题是在

ArrayList idList

中，当您从上一个

B

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

调用一行，该行销毁id为“B”的

活动
，这是堆栈中的2。
同时删除
B
，并按下
C
。当您完成
C
并且堆栈中没有
B
，但其Id可用时，它的
ArrayList idList
，您将获得它的
活动
，如果找不到它，它的
活动

就这些

id+=System.currentTimeMillis();

在这行代码中，每次使用系统的当前时间开始新的
活动时，您都会分配一个唯一的id。
我在这里找到了更好的解决方案：


如果您正在执行
id+System.currentTimeMillis（）
操作，您可能会遇到以下问题：

有时a+b=c+d，所以你仍然获得NPE的可能性很小
例如，您需要将id存储在某个位置才能关闭它
您应该包含整个代码，但如果没有。TabGroupActivity.java中的第46行是什么？第46行是Intent lastIntent=lastActivity.getIntent（）；我将继续添加一些代码：）事情是这样的，正如我提到的。它在崩溃前可能工作了20次，这让我相信还有别的东西。它也不总是在同一步发生崩溃。您似乎在使用putExtra传递一些参数。检查您是否在各自的活动中使用Bundle接收它们（我正在这样做：）问题似乎是，如果您第一次回答“未知”，则同一步骤可能会发生两次。使我的唯一id不再那么独特。当您按下back键并销毁具有给定id的活动时，您将同时销毁这两个活动，使其在堆栈中的较早位置到达同一活动时不可用。这似乎解决了问题。是什么让你认为这解决了问题？我想这就是问题所在。我没有考虑过，因为我真的不知道它会发生两次。但谢谢你的帮助：）标记为解决问题的答案。
manager.destroyActivity( idList.get( index ), true );

id+=System.currentTimeMillis();