java - 错误 : java. lang.NullPointerException:尝试对空对象引用调用接口(interface)方法 'int java.util.List.size()'

Question

当我在 AndroidStudio 中运行我的应用程序时，出现以下异常:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.leftover, PID: 5379
java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
    at android.widget.ArrayAdapter.getCount(ArrayAdapter.java:389)
    at com.baoyz.swipemenulistview.SwipeMenuAdapter.getCount(SwipeMenuAdapter.java:32)
    at android.widget.ListView.setAdapter(ListView.java:581)
    at com.baoyz.swipemenulistview.SwipeMenuListView.setAdapter(SwipeMenuListView.java:65)
    at com.example.leftover.StorageFragment.onCreateView(StorageFragment.java:52)
    at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2600)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:881)
    at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManagerImpl.java:1238)
    at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManagerImpl.java:1303)
    at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:439)
    at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManagerImpl.java:2079)
    at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManagerImpl.java:1869)
    at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManagerImpl.java:1824)
    at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManagerImpl.java:1727)
    at androidx.fragment.app.FragmentManagerImpl.dispatchStateChange(FragmentManagerImpl.java:2663)
    at androidx.fragment.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManagerImpl.java:2613)
    at androidx.fragment.app.FragmentController.dispatchActivityCreated(FragmentController.java:246)
    at androidx.fragment.app.FragmentActivity.onStart(FragmentActivity.java:542)
    at androidx.appcompat.app.AppCompatActivity.onStart(AppCompatActivity.java:201)
    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1435)
    at android.app.Activity.performStart(Activity.java:8013)
    at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3366)
    at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:221)
    at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:201)
    at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:173)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2044)
    at android.os.Handler.dispatchMessage(Handler.java:106)
    at android.os.Looper.loop(Looper.java:223)
    at android.app.ActivityThread.main(ActivityThread.java:7476)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:549)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:939)

这是我工作的 fragment :

public class StorageFragment extends Fragment{

private static final String TAG = "StorageFragment";

private User user;
private Button addButton; 
private AutoCompleteTextView autoEditText; 
private SwipeMenuListView listView; 
private ArrayList<String> list; 
private ArrayAdapter listAdapter; 
private ArrayAdapter<String> autoCompleteAdapter; 
private static final String[] LEBENSMITTEL = new String[]{"Apfel", "Ananas", "Aubergine", "Alkohol", "Artischocke", "Aloe Vera"}; 

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_storage, container, false);
    addButton = (Button) v.findViewById(R.id.buttonAddToStorage);
    autoEditText = (AutoCompleteTextView) v.findViewById(R.id.search_bar);
    listView = (SwipeMenuListView) v.findViewById(R.id.listView);

    list = FileHelper.readData(getActivity());
    listAdapter = new ArrayAdapter (getActivity(), android.R.layout.simple_list_item_1, list);
    listView.setAdapter(listAdapter);

    addButton.setOnClickListener((View.OnClickListener) this);

    autoCompleteAdapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, LEBENSMITTEL);
    autoEditText.setAdapter(autoCompleteAdapter);

    /**
     * Swipe-Menu with Add- and Delete-Button
     */
    SwipeMenuCreator creator = new SwipeMenuCreator() {
        @Override
        public void create(SwipeMenu menu) {
            // Add-Item
            SwipeMenuItem editItem = new SwipeMenuItem(getActivity().getApplicationContext());
            editItem.setBackground(new ColorDrawable(Color.rgb(0xC9, 0xC9, 0xCE)));
            editItem.setWidth(170);
            editItem.setIcon(R.drawable.ic_storage_add_24dp);
            menu.addMenuItem(editItem);
            // Delete-Item
            SwipeMenuItem deleteItem = new SwipeMenuItem(getActivity().getApplicationContext());
            deleteItem.setBackground(new ColorDrawable(Color.rgb(0xF9, 0x3F, 0x25)));
            deleteItem.setWidth(170);
            deleteItem.setIcon(R.drawable.ic_storage_delete_24dp);
            menu.addMenuItem(deleteItem);
        }
    };
    listView.setMenuCreator(creator);
    listView.setOnMenuItemClickListener(new SwipeMenuListView.OnMenuItemClickListener() {
        @Override
        public boolean onMenuItemClick(int position, SwipeMenu menu, int index) {
            switch (index) {
                case 0: //add
                    showMessageDialog();
                    break;
                case 1: //delete
                    //nothing happens here

                    Log.d(TAG, "onMenuItemClick: clicked item " + index);
                    break;
            }
            // false : close the menu; true : not close the menu
            return false;
        }
    });

    return v;
}

public void setUser(User user) {
    this.user = user;
}

/**
 * Function for Add-Button
 */

public void onClick(View v) {
    switch(v.getId()){
        case R.id.buttonAddToStorage:
            String itemEntered = autoEditText.getText().toString();
            listAdapter.add(itemEntered);
            autoEditText.setText("");

            FileHelper.writeData(list, getActivity());

            Toast.makeText(getActivity(), "Lebensmittel hinzugefügt", Toast.LENGTH_SHORT).show();
            break;
    }
}

/**
 * Method for Message Alert Dialog
 */
private void showMessageDialog(){
    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity(), R.style.AlertDialogTheme);
    View v = null;
    v = LayoutInflater.from(getActivity()).inflate(R.layout.fragment_storage_layout_add_dialog, (ConstraintLayout) v.findViewById(R.id.layoutDialogContainer));
    builder.setView(v);
    ((TextView) v.findViewById(R.id.textTitle)).setText(getResources().getString(R.string.dialog_message_title));
    ((TextView) v.findViewById(R.id.textMessage)).setText(getResources().getString(R.string.dialog_message));
    ((Button) v.findViewById(R.id.buttonHoldItem)).setText(getResources().getString(R.string.im_Lager_halten));
    ((Button) v.findViewById(R.id.buttonDeleteItem)).setText(getResources().getString(R.string.aus_Lager_entfernen));

    final AlertDialog alertDialog = builder.create();

    v.findViewById(R.id.buttonHoldItem).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            alertDialog.dismiss();
            Toast.makeText(getActivity(), "Ja", Toast.LENGTH_SHORT).show();
        }
    });

    v.findViewById(R.id.buttonDeleteItem).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            alertDialog.dismiss();
            Toast.makeText(getActivity(), "Nein", Toast.LENGTH_SHORT).show();
        }
    });

    if(alertDialog.getWindow() != null) {
        alertDialog.getWindow().setBackgroundDrawable(new ColorDrawable(0));
    }

    alertDialog.show();
} }

错误在 fragment 中，但我不确定。当我尝试打开应用程序时，此错误导致应用程序崩溃。

编辑:

我调试了 onCreateView() 。变量 addButton、autoEditText、listView、list、listAdapter 和 autoCompleteAdapter 均为 null。

但我就是不明白，请原谅我的盲目，但我在哪里可以改变这些不再为空。我以为我初始化了它们...:(

请帮忙。

(我知道这段代码中还有其他一些错误，但现在我需要有关此错误的帮助)

Answer 1

也许您忘记初始化 FileHelper？

FileHelper fileHelper=new FileHelper();
fileHelper.readData(getActivity());