在空obj參考上呼叫虛擬方法'voidandroidx.viewpager2.widget.ViewPager2.setAdapter(androidx.recyclerview.widge

我正在嘗試使用 BottomNaviagtionView 顯示兩個片段，一個是 HomeFragment，第二個是 AboutFragment。第一個是 Ho 和第二個 mefragment，它也有 tablayout 并包含兩個正常作業的片段。但現在我只想要來自 bottomNavigationview 1st HomeFragment 和 2nd Aboutfragment 的 2 個主要片段。

HomeFragment.java

public class HomeFragment extends Fragment {
FloatingActionButton add;
TabLayout tabLayout;
ViewPager2 viewPager2;
MyAdapter myAdapter;
public HomeFragment() {
    // Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    add = getActivity().findViewById(R.id.addButton);
    tabLayout = getActivity().findViewById(R.id.tabLayout);
    viewPager2 = getActivity().findViewById(R.id.viewPager);
    myAdapter = new MyAdapter(this);
    viewPager2.setAdapter(myAdapter);//this is where error occured
     //Attempt to invoke virtual method 'void androidx.viewpager2.widget.ViewPager2.setAdapter(androidx.recyclerview.widget.RecyclerView$Adapter)' on a null object reference
    add.setOnClickListener(view -> {
        Intent intent = new Intent(getActivity().getApplicationContext(), AddMesurement.class);
        startActivity(intent);
    });
    tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            viewPager2.setCurrentItem(tab.getPosition());
        }
        @Override
        public void onTabUnselected(TabLayout.Tab tab) {
        }
        @Override
        public void onTabReselected(TabLayout.Tab tab) {
        }
    });
    viewPager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
        @Override
        public void onPageSelected(int position) {
            super.onPageSelected(position);
            tabLayout.getTabAt(position).select();
        }
    });
    return inflater.inflate(R.layout.fragment_home, container, false);
}
}

片段主頁.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
 xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@ id/homeFragment"
tools:context=".HomeFragment">

<com.google.android.material.floatingactionbutton.FloatingActionButton
    android:id="@ id/addButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginEnd="10dp"
    android:layout_marginBottom="10dp"
    android:elevation="3dp"
    app:borderWidth="0dp"
    android:src="@drawable/ic_baseline_note_add_24"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    android:backgroundTint="@color/purple_200"
    android:contentDescription="@string/app_name" />

<com.google.android.material.tabs.TabLayout
    android:id="@ id/tabLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" >
    <com.google.android.material.tabs.TabItem
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:id="@ id/recent"
        android:text="RECENT"/>
    <com.google.android.material.tabs.TabItem
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:id="@ id/confirmed"
        android:text="CONFIRMED"/>

</com.google.android.material.tabs.TabLayout>

<androidx.viewpager2.widget.ViewPager2
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@ id/viewPager"
    app:layout_constraintTop_toBottomOf="@ id/tabLayout"/>
</androidx.constraintlayout.widget.ConstraintLayout>

//這是我的主java檔案，它有底欄，我正在從這個檔案訪問片段。 主頁.java

 public class HomePage extends AppCompatActivity {
BottomNavigationView bottomNavigationView;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home_page);
    bottomNavigationView = findViewById(R.id.bottomBar);
    bottomNavigationView.setSelectedItemId(R.id.home);
    fragmentReplace(new HomeFragment());
    bottomNavigationView.setOnItemSelectedListener(new NavigationBarView.OnItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()){
                case (R.id.home):
                    fragmentReplace(new HomeFragment());
                    break;
                case (R.id.about):
                    fragmentReplace(new AboutFragment());
                    break;
                default:
                    fragmentReplace(new HomeFragment());
                    break;
            }
            return true;
        }
    });
}
private void fragmentReplace(Fragment fragment){
    FragmentManager fragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    fragmentTransaction.replace(R.id.frameLayout, fragment);
    fragmentTransaction.commit();
}
}

uj5u.com熱心網友回復：

 @Override 
    public View onCreateView(LayoutInflater inflater, 
ViewGroup container, Bundle savedInstanceState) {
                // Inflate the layout for this fragment
                View v = inflater.inflate(R.layout.fragment_home, container, false)
                add = v.findViewById(R.id.addButton);
                tabLayout = v.findViewById(R.id.tabLayout);
                viewPager2 = v.findViewById(R.id.viewPager);
                myAdapter = new MyAdapter(this);
                viewPager2.setAdapter(myAdapter);
                add.setOnClickListener(view -> {
                    Intent intent = new Intent(getActivity().getApplicationContext(), AddMesurement.class);
                    startActivity(intent);
                });
                tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
                    @Override
                    public void onTabSelected(TabLayout.Tab tab) {
                        viewPager2.setCurrentItem(tab.getPosition());
                    }
                    @Override
                    public void onTabUnselected(TabLayout.Tab tab) {
                    }
                    @Override
                    public void onTabReselected(TabLayout.Tab tab) {
                    }
                });
                viewPager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
                    @Override
                    public void onPageSelected(int position) {
                        super.onPageSelected(position);
                        tabLayout.getTabAt(position).select();
                    }
                });
                return v; }

只需添加一些語法糖，因為這個編輯器沒有很好地粘貼代碼。

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