Question

我正在尝试水平滑动页面但我得到了NullPointerException。我试图刷3页，我在其中一个页面的textview上使用textview和onclicklistener。当我运行应用程序时，它不断显示异常。

这是我的logcat

01-08 19:07:21.400: E/AndroidRuntime(22391): FATAL EXCEPTION: main
01-08 19:07:21.400: E/AndroidRuntime(22391): java.lang.NullPointerException
01-08 19:07:21.400: E/AndroidRuntime(22391):    at com.programr.dishoom.HotelMap.showHotelMap(HotelMap.java:184)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at com.programr.dishoom.HotelMap$MyPagerAdapter.instantiateItem(HotelMap.java:131)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.support.v4.view.PagerAdapter.instantiateItem(PagerAdapter.java:110)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:801)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.support.v4.view.ViewPager.populate(ViewPager.java:930)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.support.v4.view.ViewPager.populate(ViewPager.java:881)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1366)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.view.View.measure(View.java:12937)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5045)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1369)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.widget.LinearLayout.measureVertical(LinearLayout.java:660)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.widget.LinearLayout.onMeasure(LinearLayout.java:553)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.view.View.measure(View.java:12937)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5045)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.widget.FrameLayout.onMeasure(FrameLayout.java:293)
01-08 19:07:21.400: E/AndroidRuntime(22391):    at android.view.View.measure(View.java:12937)
 01-08 19:07:21.400: E/AndroidRuntime(22391):   at android.widget.LinearLayout.measureVertical(LinearLayout.java:812)

这是我的完整代码。

 @Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_hotel_map);
    menuList =  (ListView)findViewById(R.id.listView2);        
    }        
    MyPagerAdapter adapter = new MyPagerAdapter();
     myPager = (ViewPager) findViewById(R.id.myfivepanelpager);
     myPager.setAdapter(adapter);
     myPager.setCurrentItem(3);      
}

private class MyPagerAdapter extends PagerAdapter {

    public int getCount(){
        return 3;
    }

    public Object instantiateItem(View collection, int position) {

        LayoutInflater inflater = (LayoutInflater) collection.getContext()
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        int resId = 0;
        switch (position) {
        case 0: 
            resId = showHotelContact();             
            break;
        case 1:
            resId = showHotelAddress();         
            break;              
        case 2:     
            resId = showHotelMap();             
            break;      
        }

        View view = inflater.inflate(resId, null);
        ((ViewPager) collection).addView(view, 0);
        return view;
    }

    @Override
    public void destroyItem(View arg0, int arg1, Object arg2) {
        ((ViewPager) arg0).removeView((View) arg2);

    }

    @Override
    public void finishUpdate(View arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public boolean isViewFromObject(View arg0, Object arg1) {
        return arg0 == ((View) arg1);

    }

    @Override
    public void restoreState(Parcelable arg0, ClassLoader arg1) {
        // TODO Auto-generated method stub

    }

    @Override
    public Parcelable saveState() {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public void startUpdate(View arg0) {
        // TODO Auto-generated method stub

    }

}

public int showHotelMap()
{
    int resId;
    resId = R.layout.clickformap;
TextView clickOnMap = (TextView)findViewById(R.id.clickmap);

clickOnMap.setOnClickListener(new View.OnClickListener() {

    @Override
    public void onClick(View v) {
        Intent intent = new Intent (getApplicationContext(),ShowMap.class);
        startActivity(intent);

    }
});
    return resId;
}
public int showHotelAddress()
{
    int resId;
    resId = R.layout.hoteladdress;
    return resId;
}
public int showHotelContact()
{
    int resId;
    resId = R.layout.hotelcontact;
    return resId;
}

}

我不明白为什么我会收到这个错误请给我任何提示或参考。

Answer 1

您呼叫findViewById()的上下文位于“活动”中。这会检查您通过活动的setContent()方法在onCreate()中设置的主要布局。如果TextView R.id.clickmap不是您传递给Activity的视图的一部分，则无法找到它。

Activity类本身不知道在PagerAdapter中创建/销毁的视图，因此无法找到它们。为了使findViewById()起作用，您需要在对包含该类的父视图的引用上调用它。它可以是View层次结构中的任何父级，但它必须是父级或视图本身。

编辑：

我不知道您是如何调用showHotelMap()的，但是您需要使用包含您正在查找的TextView的父视图：

View mainView = getMainView()
TextView clickOnMap = (TextView) mainView.findViewById(R.id.clickmap);
clickOnMap.setOnClickListener(......);

其中，getMainView()是一种检索所需视图引用的方法。您如何做到这取决于您的实施。

编辑2：

private class MyPagerAdapter extends PagerAdapter {

   ...

   public Object instantiateItem(View collection, int position) {

      LayoutInflater inflater = (LayoutInflater) collection.getContext()
            .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

       View returnView = null;
       switch (position) {
          case 0: 
             returnView = showHotelContact(inflater);             
             break;
          ...   
       }

       return returnView;
   }

   ...


   public View showHotelMap(Inflater inflater)
   {
      View returnView = inflater.inflate(R.layout.clickformap, null);
      TextView clickOnMap = (TextView)returnView.findViewById(R.id.clickmap);

      clickOnMap.setOnClickListener(new View.OnClickListener() {
         @Override
         public void onClick(View v) {
            Intent intent = new Intent (getApplicationContext(),ShowMap.class);
            startActivity(intent);
         }
      });
      return returnView;
   }
}

在此实施中，您要在方法中对View进行夸大，设置它，然后将其返回到PagerAdapter。您无需将其添加到ViewPager，因为ViewPager本身将调用instantiateItem()并将其添加到自身。

Answer 2

您在

中有 NullPointerException

TextView clickOnMap = (TextView)findViewById(R.id.clickmap);
clickOnMap.setOnClickListener(new View.OnClickListener()...

可能 R.id.clickmap 不是 TextView 的正确ID， findViewById 返回null clickOnMap ，然后尝试 setOnClickListener 为空对象。

Answer 3

如果您想使用findViewByID()，则需要通过其父视图引用它。

你应该使用这样的东西，

TextView clickOnMap = (TextView)ActivityName.this.findViewById(R.id.clickmap);

获取Null指针异常：不明白

3 个答案: