更改组件可见性时 Android Studio 空对象引用错误

Question

我正在制作一个应用程序，它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，某些东西是可见的，而另一些则不可见。

但是，当我尝试将组件的可见性设置为 GONE 时，我得到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。

我已经尝试在 onCreate() 方法之前的 if 子句中和 setContentView() 之后初始化 TextView 和 FragmentMenuEncomenda，我正在使用的片段的类，对象，例如 tabLayout 和 viewPager。

if 子句只是为了确保用户类型正确，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，我不知道是不是因为我试图改变 on create 方法中的可见性。

Answer 1

因为调用函数时没有创建视图。

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;
        }
    }
}

那么您可以随时拨打setIsTextViewShow 或setIsTextViewGone

 FragmentMenuEncomendas fragmentMenuEncomendas = new FragmentMenuEncomendas();
                       
 fragmentMenuEncomendas.setIsTextViewShow();//show
 fragmentMenuEncomendas.setIsTextViewGone();//gone