Android Studio加载内部存储数据错误

Question

我刚开始学习Android Studio，并且一直在研究基本笔记应用程序。我已经将加载/保存方法添加到主片段。但是当我第一次启动应用程序时，它会崩溃。如果我删除加载方法并启动应用程序，那么应用程序可以按我的需要工作再次添加加载方法并第二次启动应用程序之后，应用程序也可以正常工作。所以我认为我的加载代码存在问题，因为它会在我第一次启动时导致应用程序崩溃。您可以找到第一个屏幕的主分片代码，以及我在下面的Android Studio中收到的错误。如果你能帮我解决这个问题，我将非常感激。

MainProjectList.java片段：

public class MainProjectLister extends ListFragment implements 
     AdapterView.OnItemClickListener, 
     AdapterView.OnItemLongClickListener { 

    Button addProjectButton; 
    EditText addProjectEditText; 
    public String projectTitle; 
    Projekt newProjekt; 
    ProjektSingleton projektSingleton; 
    ArrayList<Projekt> projektArrayList; 
    ArrayAdapter<String> adapter; 
    ArrayList<Projekt> tempProjektArrayList; 

    ArrayList<String> ListOfP; 

    private final String FILENAME="testfile15.txt"; 
    private final String FILENAME2="testfile16.txt"; 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     ListOfP = new ArrayList<String>(); 
     projektSingleton = ProjektSingleton.getInstance(); 
     projektSingleton.setProjektList(getSavedArrayList()); 
     ListOfP = getSavedStringList(); 

     ((AppCompatActivity) getActivity()).getSupportActionBar().setDisplayHomeAsUpEnabled(false); 
     setHasOptionsMenu(true); 

    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){ 

     View view = inflater.inflate(R.layout.main_project_lister_layout, container, false); 

     addProjectButton = (Button) view.findViewById(R.id.addProjectButton); 
     addProjectEditText = (EditText) view.findViewById(R.id.addProjectEditText); 

//addProjectButton onClck method 

     addProjectButton.setOnClickListener(new View.OnClickListener(){ 
      @Override 
      public void onClick(View view){ 

//Checking if nothing is entered in the EditText and gives Error messaga 
       if(TextUtils.isEmpty(addProjectEditText.getText().toString().trim())){ 
        Toast.makeText(getActivity(), "Please type details", Toast.LENGTH_SHORT).show(); 
        return; 
       } 

       projectTitle = addProjectEditText.getText().toString(); 
       newProjekt = new Projekt(projectTitle); 
       projektArrayList = projektSingleton.getProjectList(); 
       projektArrayList.add(newProjekt); 

       ListOfP.add(projectTitle); 
       adapter.notifyDataSetChanged(); 
       saveArrayList(projektArrayList); 
       saveStringList(ListOfP); 
       addProjectEditText.setText(""); 

      } 

     }); 

     getActivity().setTitle("DearBear Notes App"); 
     return view; 

    } 

    @Override 
    public void onActivityCreated(Bundle savedInstanceState) { 
     super.onActivityCreated(savedInstanceState); 

     adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, ListOfP); 
     setListAdapter(adapter); 

     getListView().setOnItemClickListener(this); 
     getListView().setOnItemLongClickListener(this); 
    } 

    @Override 
    public void onItemClick(AdapterView<?> adapterView, View view, int i, long l){ 

     MainProjectLister frag = new MainProjectLister(); 
     ProjectListFragmentChanger fragInterface = (ProjectListFragmentChanger) getActivity(); 
     fragInterface.projectListChangeFragment(frag); 

     projektSingleton.setProjectNumber(i); 

    } 

    @Override 
    public boolean onItemLongClick(AdapterView<?> adapterView, View view, int i, long l){ 
     projektArrayList = projektSingleton.getProjectList(); 
     projektArrayList.remove(i); 
     ListOfP.remove(i); 
     adapter.notifyDataSetChanged(); 
     saveArrayList(projektArrayList); 
     saveStringList(ListOfP); 

     return true; 
    } 

    //Interface 

    public interface ProjectListFragmentChanger{ 

     public void projectListChangeFragment(Fragment fragment); 

    } 

//Save File Method 
    public void saveArrayList(ArrayList<Projekt> arrayList) { 
     try { 
      FileOutputStream fos = getActivity().openFileOutput(FILENAME, Context.MODE_PRIVATE); 
      ObjectOutputStream out = new ObjectOutputStream(fos); 
      out.writeObject(arrayList); 
      out.close(); 
      fos.close(); 

     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
    } 

    public void saveStringList(ArrayList<String> arrayList) { 
     try { 
      FileOutputStream fos = getActivity().openFileOutput(FILENAME2, Context.MODE_PRIVATE); 
      ObjectOutputStream out = new ObjectOutputStream(fos); 
      out.writeObject(arrayList); 
      out.close(); 
      fos.close(); 

     } catch (IOException e) { 
      e.printStackTrace(); 
     } 
    } 


//Load Text File Method 

    public ArrayList<Projekt> getSavedArrayList() { 
     ArrayList<Projekt> savedArrayList = null; 

     try { 
      FileInputStream fis = getActivity().openFileInput(FILENAME); 
      ObjectInputStream ois = new ObjectInputStream(fis); 
      savedArrayList = (ArrayList<Projekt>) ois.readObject(); 
      ois.close(); 
      fis.close(); 

     } catch (IOException | ClassNotFoundException e) { 
      e.printStackTrace(); 
     } 

     return savedArrayList; 
    } 

    public ArrayList<String> getSavedStringList() { 
     ArrayList<String> savedArrayList = null; 
     try { 
      FileInputStream fis = getActivity().openFileInput(FILENAME2); 
      ObjectInputStream ois = new ObjectInputStream(fis); 
      savedArrayList = (ArrayList<String>) ois.readObject(); 
      ois.close(); 
      fis.close(); 
     } catch (IOException | ClassNotFoundException e) { 
      e.printStackTrace(); 
     } 
     return savedArrayList; 
    } 
} 

错误：

$ adb shell am start -n "com.example.lenovo.projectlist/com.example.lenovo.projectlist.MainActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER 
Client not ready yet..Waiting for process to come online 
Connected to process 2394 on device Nexus_5_API_24 [emulator-5554] 
I/art: Not late-enabling -Xcheck:jni (already on) 
W/art: Unexpected CPU variant for X86 using defaults: x86 
W/System: ClassLoader referenced unknown path: /data/app/com.example.lenovo.projectlist-2/lib/x86 
I/InstantRun: Instant Run Runtime started. Android package is com.example.lenovo.projectlist, real application class is null. 
I/art: Debugger is no longer active 
I/art: Starting a blocking GC Instrumentation 
W/System: ClassLoader referenced unknown path: /data/app/com.example.lenovo.projectlist-2/lib/x86 
W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable 
W/System.err: java.io.FileNotFoundException: /data/user/0/com.example.lenovo.projectlist/files/testfile15.txt (No such file or directory) 
W/System.err:  at java.io.FileInputStream.open(Native Method) 
W/System.err:  at java.io.FileInputStream.<init>(FileInputStream.java:146) 
W/System.err:  at android.app.ContextImpl.openFileInput(ContextImpl.java:485) 
W/System.err:  at android.content.ContextWrapper.openFileInput(ContextWrapper.java:186) 
W/System.err:  at com.example.lenovo.projectlist.MainProjectLister.getSavedArrayList(MainProjectLister.java:189) 
W/System.err:  at com.example.lenovo.projectlist.MainProjectLister.onCreate(MainProjectLister.java:53) 
W/System.err:  at android.app.Fragment.performCreate(Fragment.java:2335) 
W/System.err:  at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:949) 
W/System.err:  at android.app.BackStackRecord.setLastIn(BackStackRecord.java:857) 
W/System.err:  at android.app.BackStackRecord.calculateFragments(BackStackRecord.java:880) 
W/System.err:  at android.app.BackStackRecord.run(BackStackRecord.java:727) 
W/System.err:  at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578) 
W/System.err:  at android.app.FragmentController.execPendingActions(FragmentController.java:371) 
W/System.err:  at android.app.Activity.performStart(Activity.java:6680) 
W/System.err:  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609) 
W/System.err:  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
W/System.err:  at android.app.ActivityThread.-wrap12(ActivityThread.java) 
W/System.err:  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
W/System.err:  at android.os.Handler.dispatchMessage(Handler.java:102) 
W/System.err:  at android.os.Looper.loop(Looper.java:154) 
W/System.err:  at android.app.ActivityThread.main(ActivityThread.java:6077) 
W/System.err:  at java.lang.reflect.Method.invoke(Native Method) 
W/System.err:  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
W/System.err:  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
W/System.err: java.io.FileNotFoundException: /data/user/0/com.example.lenovo.projectlist/files/testfile16.txt (No such file or directory) 
W/System.err:  at java.io.FileInputStream.open(Native Method) 
W/System.err:  at java.io.FileInputStream.<init>(FileInputStream.java:146) 
W/System.err:  at android.app.ContextImpl.openFileInput(ContextImpl.java:485) 
W/System.err:  at android.content.ContextWrapper.openFileInput(ContextWrapper.java:186) 
W/System.err:  at com.example.lenovo.projectlist.MainProjectLister.getSavedStringList(MainProjectLister.java:208) 
W/System.err:  at com.example.lenovo.projectlist.MainProjectLister.onCreate(MainProjectLister.java:54) 
W/System.err:  at android.app.Fragment.performCreate(Fragment.java:2335) 
W/System.err:  at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:949) 
W/System.err:  at android.app.BackStackRecord.setLastIn(BackStackRecord.java:857) 
W/System.err:  at android.app.BackStackRecord.calculateFragments(BackStackRecord.java:880) 
W/System.err:  at android.app.BackStackRecord.run(BackStackRecord.java:727) 
W/System.err:  at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578) 
W/System.err:  at android.app.FragmentController.execPendingActions(FragmentController.java:371) 
W/System.err:  at android.app.Activity.performStart(Activity.java:6680) 
W/System.err:  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609) 
W/System.err:  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
W/System.err:  at android.app.ActivityThread.-wrap12(ActivityThread.java) 
W/System.err:  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
W/System.err:  at android.os.Handler.dispatchMessage(Handler.java:102) 
W/System.err:  at android.os.Looper.loop(Looper.java:154) 
W/System.err:  at android.app.ActivityThread.main(ActivityThread.java:6077) 
W/System.err:  at java.lang.reflect.Method.invoke(Native Method) 
W/System.err:  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
W/System.err:  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
D/AndroidRuntime: Shutting down VM 


        --------- beginning of crash 
E/AndroidRuntime: FATAL EXCEPTION: main 
        Process: com.example.lenovo.projectlist, PID: 2394 
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.lenovo.projectlist/com.example.lenovo.projectlist.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference 
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646) 
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
         at android.app.ActivityThread.-wrap12(ActivityThread.java) 
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
         at android.os.Handler.dispatchMessage(Handler.java:102) 
         at android.os.Looper.loop(Looper.java:154) 
         at android.app.ActivityThread.main(ActivityThread.java:6077) 
         at java.lang.reflect.Method.invoke(Native Method) 
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
        Caused by: 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:344) 
         at android.widget.ListView.setAdapter(ListView.java:493) 
         at android.app.ListFragment.setListAdapter(ListFragment.java:240) 
         at com.example.lenovo.projectlist.MainProjectLister.onActivityCreated(MainProjectLister.java:111) 
         at android.app.Fragment.performActivityCreated(Fragment.java:2361) 
         at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1014) 
         at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1171) 
         at android.app.BackStackRecord.run(BackStackRecord.java:815) 
         at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1578) 
         at android.app.FragmentController.execPendingActions(FragmentController.java:371) 
         at android.app.Activity.performStart(Activity.java:6680) 
         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2609) 
         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)  
         at android.app.ActivityThread.-wrap12(ActivityThread.java)  
         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)  
         at android.os.Handler.dispatchMessage(Handler.java:102)  
         at android.os.Looper.loop(Looper.java:154)  
         at android.app.ActivityThread.main(ActivityThread.java:6077)  
         at java.lang.reflect.Method.invoke(Native Method)  
         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)  
         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)  
Application terminated. 

Answer 1

的第一个错误是因为你还没有被创建一个文件调用openFileInput()。但是，由于您发现该错误，因此getSavedArrayList()返回null，并将其指定给ListOfP，当您尝试将null分配给ArrayAdapter时，会导致NullPointerException。

为防止出现这种情况，您应该先检查文件是否存在，然后将您的列表分配给空白列表以防止出现这两种错误;像这样：

public ArrayList<Projekt> getSavedArrayList() { 
    ArrayList<Projekt> savedArrayList = null; 
    try { 
     // get the file and check if it exists before trying to read it 
     File savedFile = new File(getActivity().getFilesDir(), FILENAME); 
     if (savedFile.exists()) { 
      // if the file containing the list exists, read it and return the list 
      FileInputStream fis = getActivity().openFileInput(FILENAME); 
      ObjectInputStream ois = new ObjectInputStream(fis); 
      savedArrayList = (ArrayList<Projekt>) ois.readObject(); 
      ois.close(); 
      fis.close(); 
     } else { 
      // if it doesn't exist, return an empty list 
      savedArrayList = new ArrayList<>(); 
     } 
    } catch (IOException | ClassNotFoundException e) { 
     e.printStackTrace(); 
    } 
    return savedArrayList; 
}