我如何在android中保存arraylist？

Question

我試圖存儲在onstop()的ArrayList並通過調用啓動getlist()恢復過，但是這似乎並沒有工作：沒有得到恢復，不知道它的存儲，即使是我的代碼是否正確？

public class Schedule extends Activity { 
Context context = this; 
ArrayList<appointment> listoffappointments = new ArrayList<appointment>(); 
ArrayList<appointment> myArray; 
ArrayList<Calendar> listofcalenders = new ArrayList<Calendar>(); 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_appointments); 
instance =this; 
listView = (ListView) findViewById(R.id.myDataListListView) ;  
    refresh(); 
} 

    public void refresh() 
    { 

     adapter = new ArrayAdapter(instance,R.layout.mylist, listoffappointments) ; 

     listView.setAdapter(adapter) ; 
    } 


@overide 
protected void onResume() { 

    FileInputStream input = null; 
    ObjectInputStream inputob = null; 
    try { 
     input = openFileInput("app.ser"); 
     inputob = new ObjectInputStream(input); 
     myArray = (ArrayList<appointment>) inputob.readObject(); 




// listoffappointments.addAll(myArray); 
THIS IS WHERE I NEED HELP I WANT THE INITIAL LIST LISTOFFAPPOINTMENTS TO HAVE ALL THE ELEMENTS OF myArray when i print each element in myArray onclose the elements i add are present but when i do listoffappointments.addall(myArray); i get a null pointerexception this is why i have the code commented 


     inputob.close(); 
     input.close(); 


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

    super.onResume(); 
} 


@Override 
protected void onStop() { 



    try { 
     FileOutputStream file =openFileOutput("app.ser", Context.MODE_PRIVATE); 
     ObjectOutputStream oos= new ObjectOutputStream(file); 
     oos.writeObject(listoffappointments); 
     oos.close(); 
     file.close(); 
    } catch (FileNotFoundException e) { 
     e.printStackTrace(); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 

    for(appointment b : myArray) 
    { 
     System.out.println(b.gettext()); 
    } 


    super.onStop(); 
} 

請大家看看評論在我的代碼的onResume（）

Answer 1

首先，我認爲你應該添加一個serialVersionUID您Serializable對象。這是未來兼容性的良好做法。第二，你可能想要大寫你的對象名（Appointment）;或者你可能想要大寫你的對象名（Appointment）;這是公約。

第三，你應該讀/從/到您的應用程序的文件目錄的寫你的對象。您使用Context.getFilesDir()獲取該目錄。這是一個什麼樣的代碼可能看起來像一個例子：

private List<Appointment> mAppointments = new ArrayList<Appointment>(); 
private List<Appointment> mListOfAppointments = new ArrayList<Appointment>(); 

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

    mFile = new File(getActivity().getFilesDir(), "MyFile.ser"); 

} 


@Override 
public void onResume() { 
    super.onResume(); 

    try { 
     ObjectInputStream ois = new ObjectInputStream(new FileInputStream(mFile)); 
     mAppointments = (List<Appointment>) ois.readObject(); 
     ois.close(); 

     if(mAppointments != null && !mAppointments.isEmpty()){ 
      mListOfAppointments.clear(); 
      mListOfAppointments.addAll(mAppointments); 
     } 

     Log.i(TAG, mAppointments.toString()); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } catch (ClassNotFoundException e) { 
     e.printStackTrace(); 
    } 
} 


@Override 
public void onStop() { 
    super.onStop(); 

    try { 
     ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(mFile)); 

     List<Appointment> list = new ArrayList<Appointment>(); 
     Appointment appointment = new Appointment(Calendar.getInstance(), "The Calendar"); 
     list.add(appointment); 

     oos.writeObject(list); 
     oos.close(); 
    } catch (FileNotFoundException e) { 
     e.printStackTrace(); 
    } catch (IOException e) { 
     e.printStackTrace(); 
    } 
} 

上面的代碼中onStop()寫入List<Appointment>到文件MyFile.ser。在onResume()中，它將List讀回到內存中。

請確認您檢查，在onResume()，那mAppointmentsIS NOT NULL因爲這將是空你第一次打開你的應用程序（只是因爲你還沒有創建「app.ser」，但 - 你不直到第一個onStop()）。你的listoffappointments列表是空的第一次，但在隨後的應用開口，所提供的第一onStop()，onResume()在約會列表不是空會發現文件和適當填充任命名單。