更改组件可见性时发生Android studio空对象引用错误

Question

我正在制作一个应用程序，为不同的用户显示相同的菜单，每个菜单都是一个片段，但根据用户的不同，某些内容是可见的，而其他内容则不可见。

然而，当我试图将组件的可见性设置为GONE时，我得到了一个空指针异常错误，我仍在测试，所以我只在片段xml上使用textView。

我已经尝试过初始化TextView和FragmentMenuEncomenda(我正在使用的片段的类(对象，它们都在onCreate()方法之前的if子句中，以及在setContentView()之后，比如tabLayout和viewPager。

if子句只是为了确保用户类型正确，并且有问题的代码如下：

public class PaginaInicialCliente extends AppCompatActivity {
private SharedPreferences sharedpreferences;
private TabLayout tabLayout;
private ViewPager viewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_pagina_inicial_cliente);
tabLayout = findViewById(R.id.tabLayoutCliente);
viewPager = findViewById(R.id.viewPagerCliente);
ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager());
//fragments a adicionar
adapter.addFragment(new FragmentMenuEncomendas(), "Encomendas");
adapter.addFragment(new FragmentMenuOpcoes(), "Opções");
//inicializar o tab layout
viewPager.setAdapter(adapter);
tabLayout.setupWithViewPager(viewPager);
sharedpreferences = getSharedPreferences("user", Context.MODE_PRIVATE);
if (sharedpreferences.getInt("tipo",0)==3) {
Log.d("USER_DEBUG","Cliente");
FragmentMenuEncomendas fragmentMenuEncomendas = new FragmentMenuEncomendas();

//THE ERROR IS HERE
fragmentMenuEncomendas.getActivity().findViewById(R.id.textViewEncomendas).setVisibility(View.GONE);
}
}
}

FragmentMenuEncomenda类：

public class FragmentMenuEncomendas extends Fragment {
View v;
public FragmentMenuEncomendas(){
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
v = inflater.inflate(R.layout.encomendas_fragment, container, false);
return v;
}
}

xml文件：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/textViewEncomendas"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/app_name"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

我搜索的所有答案都不起作用，我不知道这是否是因为我试图更改on create方法内部的可见性。

Answer 1

因为在调用函数时不会创建视图。

public class FragmentMenuEmpresas extends Fragment {
View v;
View textView;
boolean isTextViewShow = false;
boolean isViewCreated = false;
public FragmentMenuEmpresas() {
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
v = inflater.inflate(R.layout.empresas_fragment, container, false);
textView = v.findViewById(R.id.textViewEncomendas);
return v;
}
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if (isTextViewShow) {
textView.setVisibility(View.VISIBLE);
} else {
textView.setVisibility(View.GONE);
}
isViewCreated = true;
}
void setIsTextViewShow() {
if (isViewCreated) {
textView.setVisibility(View.VISIBLE);
} else {
isTextViewShow = true;
}
}
void setIsTextViewGone() {

if (isViewCreated) {
textView.setVisibility(View.GONE);
} else {
isTextViewShow = false;
}
}
}

然后你可以随时拨打setIsTextViewShow或setIsTextViewGone

FragmentMenuEncomendas fragmentMenuEncomendas = new FragmentMenuEncomendas();

fragmentMenuEncomendas.setIsTextViewShow();//show
fragmentMenuEncomendas.setIsTextViewGone();//gone