Question

我有一个导航抽屉活动，我附加了不同的片段

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener, rp_fragment.OnFragmentInteractionListener, sensors_fragment.OnFragmentInteractionListener, stats_fragment.OnFragmentInteractionListener {

我试图修改片段中包含的textview的text属性但是我一直在视图上得到一个空指针异常（java.lang.NullPointerException：尝试调用虚方法＆＃39; void com .teicrete.alucard.sempi.sensors_fragment.ctv_tvst（int）＆＃39;在空对象引用上。）

Mainactivity：

public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();
        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        if (id == R.id.nav_camera) {
            ft.replace(R.id.fragment_placeholder, new rp_fragment());
            ft.commit();
        } else if (id == R.id.nav_gallery) {
            ft.replace(R.id.fragment_placeholder, new sensors_fragment());
            ft.commit();
            gentempdata();
            sensors_fragment sf = (sensors_fragment)fm.findFragmentById(R.id.fragment_sensors);
           sf.ctv_tvst(1);

片段：

    public class sensors_fragment extends Fragment {
            // TODO: Rename parameter arguments, choose names that match
            // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
            private static final String ARG_PARAM1 = "param1";
            private static final String ARG_PARAM2 = "param2";

            TextView ctv_tv;
            TextView otv_tv;
            View view;
    ...

        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                    Bundle savedInstanceState) {
            // Inflate the layout for this fragment
            view =  inflater.inflate(R.layout.fragment_sensors, container, false);
            TextView ctv_tv = (TextView)view.findViewById(R.id.ctv_tv);
            TextView otv_tv = (TextView)view.findViewById(R.id.otv_tv);
            //ctv_tv.setText("Test");   Only this works
            return view;
        }    
...

        public void ctv_tvst(int mtext) {
            ctv_tv.setText(mtext);
        }

我在这里看过类似的帖子，但我无法解决我的问题。我能够修改textview的唯一地方是片段的onCreateView（参见上面的代码）。如果我尝试从mainactivity或其他任何地方做到这一点我得到空指针错误。

任何见解？

编辑：请在android_griezmann的帖子中查看我的回复。现在，我正在做我需要做的一切，在片段类本身，然后我加载它们。我仍然想弄清楚为什么我无法从外部访问其方法或视图。

Answer 1

从onCreateView更改这些行：

 TextView ctv_tv = (TextView)view.findViewById(R.id.ctv_tv);
 TextView otv_tv = (TextView)view.findViewById(R.id.otv_tv);

要：

 ctv_tv = (TextView)view.findViewById(R.id.ctv_tv);
 otv_tv = (TextView)view.findViewById(R.id.otv_tv);

目前您正在TextView ctv_tv分配对新对象的引用，但实际上您希望将此引用分配到类字段，这样您就不必添加TextView。

Answer 2

您的问题是Fragment本身不是TextView。您将片段对象视为null而不是textview。

因此，尝试找到如下所示的片段。

if (id == R.id.nav_gallery) {
        ft.replace(R.id.fragment_placeholder, new sensors_fragment(),"YOUR_TAG_NAME");
        ft.commit();
        gentempdata();
        sensors_fragment sf = (sensors_fragment) fm.findFragmentByTag("YOUR_TAG_NAME");
        sf.ctv_tvst(1);
    }

也改变这个方法！

public void ctv_tvst(int mtext) {
    ctv_tv.setText("" + mtext);
}

Answer 3

替换以下代码

public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                     Bundle savedInstanceState) {
     // Inflate the layout for this fragment
     view =  inflater.inflate(R.layout.fragment_sensors,container, false);
     TextView ctv_tv = (TextView)view.findViewById(R.id.ctv_tv);
     TextView otv_tv = (TextView)view.findViewById(R.id.otv_tv);
     //ctv_tv.setText("Test");   Only this works
     return view;
}

带

public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                         Bundle savedInstanceState) {
         // Inflate the layout for this fragment
         view =  inflater.inflate(R.layout.fragment_sensors,container, false);
         ctv_tv = (TextView)view.findViewById(R.id.ctv_tv);
         otv_tv = (TextView)view.findViewById(R.id.otv_tv);
         //ctv_tv.setText("Test");   Only this works
         return view;
    }

创建变量String ctv_tv_string并指定ctv_tvst()方法中的值，不要分配给TextView。它不会引发错误，因为你在oncreateview ctv_tv.setText(ctv_tv_string)

之后没有引用任何ui元素

Answer 4

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

TextView ctv_tv = (TextView)getView().findViewById(R.id.ctv_tv);
TextView otv_tv = (TextView)getView().findViewById(R.id.otv_tv);

        }

覆盖创建的onactivity，然后引用你的视图..事实上，它是获取引用你的视图的最佳位置，而不会获得NullPointer异常

Answer 5

如果您想在替换或添加时传递数据，则在片段构造函数中传递数据，如下所示

else if (id == R.id.nav_gallery) {

        ft.replace(R.id.fragment_placeholder, new sensors_fragment("your data"));
        ft.commit();

然后在你的fragmet中根据构造函数的数据更改textview值

Answer 6

这是问题所在。只需删除 TextView

即可

 public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        view =  inflater.inflate(R.layout.fragment_sensors, container, false);
        **TextView** ctv_tv = (TextView)view.findViewById(R.id.ctv_tv);
        **TextView** otv_tv = (TextView)view.findViewById(R.id.otv_tv);
        //ctv_tv.setText("Test");   Only this works
        return view;
    }

Android - 访问片段中的视图

6 个答案: