Android - ArrayAdapter，ArrayList和文件编写器未正确更新

Question

以下是我尝试创建的Android应用中的活动代码。它是一个＆＃34;待办事项列表＆＃34;可以通过动态调整（添加/删除）并保存到文件中。在测试时，我发现以下错误（1）我似乎无法纠正，我有一个问题（2）：

1. 前几个＆＃34;添加＆＃34;列表显示为重复。删除adaptor.add vs list.add没有修复它。如果我离开视图并返回，则在您添加＆＃34;时复制。停止。但是，列表存储的文件具有重复项。当我尝试＆＃34;删除＆＃34;列表中的项目，但我认为重复导致问题。如何停止复制？
2. 如何使用视图中的当前列表完全覆盖文件中的数据？

public class ToDo extends ListActivity {

/** Items entered by the user is stored in this ArrayList variable */
ArrayList list = new ArrayList();

/** Declaring an ArrayAdapter to set items to ListView */
ArrayAdapter adapter;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    /** Setting a custom layout for the list activity */
    setContentView(R.layout.content_to_do);

    /** Reference to the add button of the layout main.xml */
    Button btn = (Button) findViewById(R.id.btnAdd);

    /** Reference to the delete button of the layout main.xml */
    Button btnDel = (Button) findViewById(R.id.btnDel);

    Button btnClr = (Button) findViewById(R.id.btnClr);

    /** Defining the ArrayAdapter to set items to ListView */

    adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_multiple_choice, list);

    File file = new File(Environment.getExternalStoragePublicDirectory(
            Environment.DIRECTORY_DOWNLOADS), "TODOList.ser");
    if(file.exists()) {
        if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {
            try {
                // create a file in downloads directory
                FileInputStream fis = new FileInputStream(
                        new File(Environment.getExternalStoragePublicDirectory(
                                Environment.DIRECTORY_DOWNLOADS), "TODOList.ser")
                );
                ObjectInputStream is = new ObjectInputStream(fis);
                list = (ArrayList<String>) is.readObject();
                System.out.println(list);

                Iterator<String> it = list.iterator();
                while (it.hasNext()) {
                    System.out.println("Data is "+ it.next());
                    adapter.add(it.next());
                }

                is.close();
                Log.v("MyApp","File has been read");
            } catch(Exception ex) {
                ex.printStackTrace();
                Log.v("MyApp","File wasn't read");
            }
        }
    }
    /** Defining a click event listener for the button "Add" */
    OnClickListener listener = new OnClickListener() {
        @Override
        public void onClick(View v) {
            EditText edit = (EditText) findViewById(R.id.txtItem);
            list.add(edit.getText().toString());
            adapter.add(edit.getText().toString());
            edit.setText("");
            adapter.notifyDataSetChanged();

            if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {
                try {
                    // create a file in downloads directory
                    FileOutputStream fos =
                            new FileOutputStream(
                                    new File(Environment.getExternalStoragePublicDirectory(
                                            Environment.DIRECTORY_DOWNLOADS), "TODOList.ser")
                            );
                    ObjectOutputStream os = new ObjectOutputStream(fos);
                    os.writeObject(list);
                    os.close();
                    Log.v("MyApp","File has been written");
                } catch(Exception ex) {
                    ex.printStackTrace();
                    Log.v("MyApp","File didn't write");
                }
            }

        }
    };

    /** Defining a click event listener for the button "Delete" */
    OnClickListener listenerDel = new OnClickListener() {
        @Override
        public void onClick(View v) {
            /** Getting the checked items from the listview */
            SparseBooleanArray checkedItemPositions = getListView().getCheckedItemPositions();
            int itemCount = getListView().getCount();

            for(int i=itemCount-1; i >= 0; i--){
                if(checkedItemPositions.get(i)){
                    adapter.remove(list.get(i));
                    list.remove(i);   //try to adjust array list before write
                }
            }
            checkedItemPositions.clear();
            adapter.notifyDataSetChanged();
            if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {
                try {
                    // create a file in downloads directory
                    FileOutputStream fos =
                            new FileOutputStream(
                                    new File(Environment.getExternalStoragePublicDirectory(
                                            Environment.DIRECTORY_DOWNLOADS), "TODOList.ser")
                            );
                    ObjectOutputStream os = new ObjectOutputStream(fos);
                    os.writeObject(list);
                    os.close();
                    Log.v("MyApp","File has been written");
                } catch(Exception ex) {
                    ex.printStackTrace();
                    Log.v("MyApp","File didn't write");
                }
            }
        }
    };


    /** Defining a click event listener for the button "Delete" */
    OnClickListener listenerClr = new OnClickListener() {
        @Override
        public void onClick(View v) {
            if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {
                try {
                    File clearit = new File(Environment.getExternalStoragePublicDirectory(
                                            Environment.DIRECTORY_DOWNLOADS), "TODOList.ser");
                    clearit.delete();
                    adapter.clear();
                    Log.v("MyApp","File has been deleted");
                } catch(Exception ex) {
                    ex.printStackTrace();
                    Log.v("MyApp","File didn't write");
                }
            }
        }
    };


    /** Setting the event listener for the add button */
    btn.setOnClickListener(listener);

    /** Setting the event listener for the delete button */
    btnDel.setOnClickListener(listenerDel);

    /** Setting the event listener for the clear button */
    btnClr.setOnClickListener(listenerClr);

    /** Setting the adapter to the ListView */
    setListAdapter(adapter);

    }

}