尝试在空对象引用上调用虚拟方法“void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)”[重复]

Question

所以基本上当我运行我的应用程序时，该应用程序不会给我任何错误但是，当我尝试在我的应用程序中打开一个名为“查找食物捐赠”的页面时，它在日志中给了我这个错误代码：

'Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference' and also said that its Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.recyclerview.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference: 

在 com.example.myddproject.FindFoodDonations.onCreate(FindFoodDonations.java:66)。

下面是Java文件中出现错误的应用程序代码（行加粗）

public class FindFoodDonations extends AppCompatActivity{
    private DatabaseReference ProductsRef;
    private RecyclerView recyclerView;
    RecyclerView.LayoutManager layoutManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_find_food_donations);

        ProductsRef = FirebaseDatabase.getInstance().getReference().child("Products");

        BottomNavigationView bottomNavigationView = findViewById(R.id.bottom_organization_navigation);
        bottomNavigationView.setSelectedItemId(R.id.nav_find_donations);
        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                switch (item.getItemId()) {
                    case R.id.nav_find_donations:
                        return true;
                    case R.id.nav_organization_home:
                        startActivity(new Intent(getApplicationContext()
                                , HomeOrganization.class));
                        overridePendingTransition(0, 0);
                        return true;
                    case R.id.nav_organization_logout:
                        FirebaseAuth.getInstance().signOut();
                        Intent intosignup = new Intent(FindFoodDonations.this, MainActivity.class);
                        startActivity(intosignup);
                        return true;
                }
                return false;
            }
        });
        recyclerView = findViewById(R.id.recycler_menu);
        **recyclerView.setHasFixedSize(true);**
        layoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(layoutManager);

    }

    @Override
    protected void onStart() {
        super.onStart();

        FirebaseRecyclerOptions<Product> options =
                new FirebaseRecyclerOptions.Builder<Product>()
                        .setQuery(ProductsRef, Product.class)
                        .build();


        FirebaseRecyclerAdapter<Product, ProductViewHolder> adapter =
                new FirebaseRecyclerAdapter<Product, ProductViewHolder>(options) {
                    @Override
                    protected void onBindViewHolder(@NonNull ProductViewHolder productViewHolder, int i, @NonNull Product product) {
                        productViewHolder.txtProductName.setText(product.getPname());
                        productViewHolder.txtProductPrice.setText("Quantity = " + product.getQuantity() + " KG's");
                        Picasso.get().load(product.getImage()).into(productViewHolder.imageView);
                    }

                    @NonNull
                    @Override
                    public ProductViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
                        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.product_items_layout, parent, false);
                        ProductViewHolder holder = new ProductViewHolder(view);
                        return holder;
                    }
                };
        recyclerView.setAdapter(adapter);
        adapter.startListening();
    }
}

上面页面的 XML 文件的代码是：

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".FindFoodDonations">
    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/bottom_organization_navigation"
        app:itemBackground="@color/design_default_color_on_primary"
        app:itemTextColor="@drawable/selector"
        app:itemIconTint="@drawable/selector"
        app:menu="@menu/organizationbottom_navigation"
        android:layout_alignParentBottom="true"/>

还有一个链接的页面（我不知道它是否需要，但我会放在这里）

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context=".HomeActivity">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_menu"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scrollbars="vertical"
        >
    </android.support.v7.widget.RecyclerView>

</RelativeLayout>

我以前没有遇到过这样的错误，所以我不知道该怎么办。

Answer 1

试试这个

<androidx.recyclerview.widget.RecyclerView
            android:id="@+id/recycler_menu"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:scrollbars="vertical"/>

代替

<android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_menu"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scrollbars="vertical"
        >
    </android.support.v7.widget.RecyclerView>

Answer 2

剪切recyclerView.setHasFixedSize(true) 并将其粘贴到此代码recyclerView.setLayoutManager(layoutManager) 下方。希望它会起作用吗？