选项卡活动中未加载第三个片段并出现错误

Question

基本上，我在viewpager中有三个片段来创建一个选项卡式活动。我的主活动中有一个名为“ SecondActivity”的按钮“ GotoCart”。该按钮的工作原理是：单击该按钮时，它将移至名为“ Cart”的新活动。在购物车中，我有一个ListView，它从每个Fragment中分别存在的三个不同的ArrayList中获取数据。现在的问题是，当我先单击“ GoToCart”按钮时，两个片段ArrayList被初始化并将其数据放入FinalList中，但显示错误对于Screen Shot of error的第三个片段，“ arrayList未初始化”。为前两个片段工作，但不为第三个片段工作，我不明白这是什么问题。请帮忙！！！。以下是第三个片段.java文件。

public class FragSandwitch extends Fragment implements View.OnClickListener,Serializable{
Button btnadd1,btnadd2,btnadd3,btnadd4,btnadd5;
static ArrayList<SingleCartItem> sandwichList;
String x,y;
TextView itemName,price;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
    final View myView = inflater.inflate(R.layout.frag_sandwich, container, false);
    btnadd1=(Button)myView.findViewById(R.id.sandwich1_btn_add);
    btnadd2=(Button)myView.findViewById(R.id.sandwich2_btn_add);
    btnadd3=(Button)myView.findViewById(R.id.sandwich3_btn_add);
    btnadd4=(Button)myView.findViewById(R.id.sandwich4_btn_add);
    btnadd5=(Button)myView.findViewById(R.id.sandwich5_btn_add);

    sandwichList = new ArrayList<SingleCartItem>();

    btnadd1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if(btnadd1.getText().toString().equals("+ADD")) {
                itemName = (TextView) myView.findViewById(R.id.sandwich1_txt_name);
                price = (TextView) myView.findViewById(R.id.sandwich1_txt_price);
                x = (String) itemName.getText();
                y = (String) price.getText();
                sandwichList.add(new SingleCartItem(x, y));
                btnadd1.setText("Added to Cart");
            }
        }
    });
    btnadd2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (btnadd2.getText().toString().equals("+ADD")) {
                itemName = (TextView) myView.findViewById(R.id.sandwich2_txt_name);
                price = (TextView) myView.findViewById(R.id.sandwich2_txt_price);
                x = (String) itemName.getText();
                y = (String) price.getText();
                sandwichList.add(new SingleCartItem(x, y));
                btnadd2.setText("Added to Cart");
            }

        }

    });
    btnadd3.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (btnadd3.getText().toString().equals("+ADD")) {
                itemName = (TextView) myView.findViewById(R.id.sandwich3_txt_name);
                price = (TextView) myView.findViewById(R.id.sandwich3_txt_price);
                x = (String) itemName.getText();
                y = (String) price.getText();
                sandwichList.add(new SingleCartItem(x, y));
                btnadd3.setText("Added to Cart");
            }
        }

    });
    btnadd4.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (btnadd4.getText().toString().equals("+ADD")){
                itemName = (TextView) myView.findViewById(R.id.sandwich4_txt_name);
                price = (TextView) myView.findViewById(R.id.sandwich4_txt_price);
                x = (String) itemName.getText();
                y = (String) price.getText();
                sandwichList.add(new SingleCartItem(x, y));
                btnadd4.setText("Added to Cart");
            }
        }

    });
    btnadd5.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if(btnadd5.getText().toString().equals("+ADD")){
                itemName = (TextView) myView.findViewById(R.id.sandwich5_txt_name);
                price = (TextView) myView.findViewById(R.id.sandwich5_txt_price);
                x=(String)itemName.getText();
                y=(String)price.getText();
                sandwichList.add(new SingleCartItem(x,y));
                btnadd5.setText("Added to Cart");
            }
        }
    });
    return myView;
}
@Override
public void onClick(View v) {

}

这是“ GoToCart”按钮的“ onClick”的“ SecondActivity”代码。

public void goToCart(View view) {


    for (int i=0;i<FragBurger.burgerList.size();i++)//working
        finalList.add(FragBurger.burgerList.get(i));

    for (int i=0;i<FragPizza.pizzaList.size();i++)//working
        finalList.add(FragPizza.pizzaList.get(i));

    for (int i=0;i<FragSandwitch.sandwichList.size();i++)//Error
        finalList.add(FragSandwitch.sandwichList.get(i));
    Intent goToCart=new Intent(SecondActivity.this,Cart.class);
    Bundle args = new Bundle();
    args.putSerializable("ARRAYLIST",(Serializable)finalList);
    goToCart.putExtra("BUNDLE",args);
    startActivity(goToCart);
}

“购物车”活动：

public class Cart extends AppCompatActivity implements View.OnClickListener{

ArrayList<SingleCartItem> arrayList;
ListView listView;
TextView quantity;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.cart);
    getSupportActionBar().setTitle("Cart");
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    listView=(ListView)findViewById(R.id.added_list_items);
    Intent intent = getIntent();
    Bundle args = intent.getBundleExtra("BUNDLE");
    arrayList = (ArrayList<SingleCartItem>) args.getSerializable("ARRAYLIST");
    MyAdapterView my_adapter_view=new MyAdapterView( getApplicationContext(),arrayList);
    my_adapter_view.notifyDataSetChanged();

    listView.setAdapter(my_adapter_view);
    args.remove("BUNDLE");}}