Android 工作室 null object 更改組件可見性時出現參考錯誤

Question

我正在制作一個應用程序，它為不同的用戶顯示相同的菜單，每個菜單都是一個片段，但根據用戶的不同，某些東西是可見的，而另一些則不是。

但是，當我嘗試將組件的可見性設置為GONE時，出現 null 指針異常錯誤，我仍在測試，因此我僅在片段 xml 上使用 textView。

我已經嘗試初始化TextView和FragmentMenuEncomenda ，我正在使用的片段的 class 對象，都在onCreate()方法之前的 if 子句內和setContentView()之后，如tabLayout和viewPager 。

if 子句只是為了確保用戶類型正確，有問題的代碼如下：

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 class：

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>

我搜索過的所有答案都沒有用，我不知道是不是因為我試圖改變 on create 方法中的可見性。

Answer 1

因為調用 function 時並沒有創建視圖。

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;
        }
    }
}

然后您可以隨時調用setIsTextViewShow或setIsTextViewGone

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