I have three linked lists called "Titles" (String linked list), "Descriptions" (String linked list), and "Pictures" (Bitmap linked list). I'm 100% sure that they are full of data. I've printed out their data just before the line MyAdapter myAdapter = new MyAdapter(this, Titles, Descriptions, Pictures); and it has exactly the data I expect ("MyAdapter" is a class). However, in the MyAdapter class, all three linked lists are null, as I get the following error:

2022-08-07 19:13:03.009 14983-14983/com.example.usshop E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.usshop, PID: 14983
    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.util.LinkedList.add(java.lang.Object)' on a null object reference
        at com.example.usshop.MyAdapter.<init>(MyAdapter.java:30)
        at com.example.usshop.Shop.CreateItems(Shop.java:70)
        at com.example.usshop.Shop$1.onDataChange(Shop.java:56)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:75)
        at com.google.firebase.database.core.view.DataEvent.fire(DataEvent.java:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(EventRaiser.java:55)
        at android.os.Handler.handleCallback(Handler.java:883)
        at android.os.Handler.dispatchMessage(Handler.java:100)
        at android.os.Looper.loop(Looper.java:237)
        at android.app.ActivityThread.main(ActivityThread.java:8167)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:496)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1100)

Below is the code of the two classes (Note: for the "Shop" class, the "CreateItems" method mainly matters because it sends the data to the other class. Included the rest in case it somehow affects the issue):

public class Shop extends AppCompatActivity {
    RecyclerView rv;
    //Below is unimportant; ignore.
/*
    String s1[], s2[];
    int images[] = {R.drawable.apple, R.drawable.banana, R.drawable.graoe,
            R.drawable.orange, R.drawable.pineapple, R.drawable.peach};
 */

    LinkedList<String> Titles = new LinkedList<>(), Descriptions = new LinkedList<>();
    LinkedList<Integer> Prices = new LinkedList<>();
    LinkedList<Bitmap> Pictures = new LinkedList<>();

    DatabaseReference ItemsReference = FirebaseDatabase.getInstance().getReference("Items");

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

        //Whenever the value of ItemsReference changes, it is stored
        ItemsReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                // Storing all of the data
                for (DataSnapshot DS : snapshot.getChildren()) {
                    BackendDataStorage TempData = DS.getValue(BackendDataStorage.class);

                    Titles.add(TempData.getTitle());
                    Descriptions.add(TempData.getDescription());
                    Prices.add(TempData.getPrice());
                    Pictures.add(Extras.StringToBitMap(TempData.getPicture()));
                }
                CreateItems();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError error) {
                Log.d("Error", "Failed to read value: "   error.getMessage());
            }
        });
    }

    public void CreateItems () {
        Log.d("index", Titles.get(0)); // Just making sure Titles isn't null
        rv = findViewById(R.id.Recycler_View);

        MyAdapter myAdapter = new MyAdapter(this, Titles, Descriptions, Pictures);
        rv.setAdapter(myAdapter);
        rv.setLayoutManager(new LinearLayoutManager(this));
    }
}

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
    private LinkedList<String> data1, data2;
    private LinkedList<Bitmap> images;
    private Context context;

    public MyAdapter (Context ct, LinkedList<String> s1, LinkedList<String> s2, LinkedList<Bitmap> img) {
        context = ct;

        // Looping through LinkedList indexes & replacing with new variables
        for (int i = 0; i < s1.size(); i  ) {
            data1.add(s1.get(i));
            data2.add(s2.get(i));
            images.add(img.get(i));
        }
    }
}

CodePudding user response：

However, in the MyAdapter class, all three linked lists are null

No, they are not. You have misread the error message. The error is not about any of the arguments you pass to the MyAdapter constructor, but about a member of the MyAdapter class.

Note for instance that there is no error on evaluating s1.size() in the for loop heading, so s1 is not null.

But the error is not about s1, s2 or img. It is about calling the add method on a null reference:

java.lang.NullPointerException: Attempt to invoke virtual method boolean java.util.LinkedList.add(java.lang.Object) on a null object reference

The line where add is called, is here:

data1.add(s1.get(i));

So, data1 is null. And this is indeed the case. Your class has a private instance member called data1 which has not been initialised. You need to do something like:

data1 = new LinkedList<String>();