ArrayList / ArrayAdapter .add() 函数覆盖最后一个元素而不是添加到数组的末尾

Question

我正在处理一项学校作业，即创建一个带有 2 个活动的书签应用程序，一个名为 BookNote 的 ListActivity 和一个名为 ManageActivity 的活动。

所有输入都必须在 ManageActivity 中完成并传回 BookNote 以进行列表更改、读取和写入数据。

我的问题是当我在 BookNote 活动中调用函数“addBookmark”时。我想要发生的是在列表末尾添加一个新列表项，其中包含书签对象信息。

发生的情况是列表末尾的任何项目都被简单地覆盖，而不是创建一个新项目。

下面是我的代码，cmets 说明了我对每个函数的意图。

BookNote 活动

public class BookNote extends ListActivity{
    private ArrayAdapter<Bookmark> adapter;
    private final String FILENAME = "bookmarks.txt";
    public String title = "EMPTY", url = "EMPTY", note = "EMPTY";
    public String bookmarkClicked ="";
    public int listViewID = 0;
    public boolean intentListener = false;
    public int intentReturnCounter = 0;
    public boolean addBookmarkClicked = false;
    public ArrayList<Bookmark> bookmarkList;


    /*When activity is called, list is populated by readData(), addBookmarkClicked is reset to false, intentLister is changed to true if returning from ManageActivity,
      if intentListener is true, addBookmarkClicked will be changed to true if AddBookmark is clicked from the BookNote menubar, a bookmark object will be created with information
       passed back from ManageActivity, and if addBookmarkClicked is true, the bookmark object will be added to the end of the list, otherwise it will be inserted in the same
       list element from which was chosen to edit.*/
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);


        bookmarkList = readData();
        adapter = new ArrayAdapter<Bookmark>(this, android.R.layout.simple_list_item_1, bookmarkList);
        setListAdapter(adapter);

        addBookmarkClicked = false;
        intentListener = false;



            intentListener = getIntent().getBooleanExtra("listen", false);

            if (intentListener == true) {
                addBookmarkClicked = getIntent().getExtras().getBoolean("addBookmarkClicked", false);
                title = getIntent().getExtras().getString("title");
                url = getIntent().getExtras().getString("url");
                note = getIntent().getExtras().getString("note");
                listViewID = getIntent().getExtras().getInt("listViewID");
                Bookmark bookmark = new Bookmark(title, url, note);
                Toast.makeText(getApplicationContext(), "Intent return: True", Toast.LENGTH_SHORT).show();
                intentListener = false;

                if (addBookmarkClicked == true) {
                    addBookmark(bookmark);
                    Toast.makeText(getApplicationContext(), "Bookmark Added", Toast.LENGTH_SHORT).show();
                    addBookmarkClicked = false;
                } else {
                    insertBookmark(bookmark, listViewID);
                    Toast.makeText(getApplicationContext(), "Bookmark Edited", Toast.LENGTH_SHORT).show();
                }
                //writeData();
        }
    }



    //reads data when app runs and fills arrayadapter and list with items saved to bookmarks.txt
    private ArrayList<Bookmark> readData(){
        ArrayList<Bookmark> bookmarks = new ArrayList<>();

        try {
            FileInputStream fis = openFileInput(FILENAME);
            Scanner scanner = new Scanner(fis);
            if (scanner.hasNext()){
                String titleScan = scanner.nextLine();
                String urlScan = scanner.nextLine();
                String noteScan = scanner.nextLine();
                Bookmark bookmark = new Bookmark(titleScan, urlScan, noteScan);
                bookmarks.add(bookmark);
            }else{
                Bookmark bookmark = new Bookmark("Example Title", "Example URL", "Example Note");
                bookmarks.add(bookmark);
            }
            scanner.close();
        } catch (FileNotFoundException e) {
        }
        return bookmarks;
    }

    private void writeData(){
        try {
            FileOutputStream fos = openFileOutput(FILENAME, Context.MODE_PRIVATE);
            OutputStreamWriter osw = new OutputStreamWriter(fos);
            BufferedWriter bw = new BufferedWriter(osw);
            PrintWriter pw = new PrintWriter(bw);

            for(int i = 0; i < adapter.getCount(); i++){
                Bookmark bookmark = adapter.getItem(i);
                pw.println(bookmark.getTitle() + "\n" + bookmark.getUrl() + "\n" + bookmark.getNote());
            }
            pw.close();
        } catch (FileNotFoundException e) {
            Log.e("Write ERR", "Cannot save: " + e.getMessage());
            e.printStackTrace();
            Toast.makeText(BookNote.this, "Error saving", Toast.LENGTH_SHORT).show();
        }
    }

    //If addBookmark menu item is clicked, this will be called to add a bookmark to the end of the ArrayAdapter.
    private void addBookmark(Bookmark bookmark){
        adapter.add(bookmark);
        writeData();
    }


    //Calls ManageActivity and reports information about app.
    public void gotoManageActivity(boolean addBookmarkClicked){
        Intent manageIntent = new Intent(this, ManageActivity.class);
        Bundle extras = new Bundle();
        extras.putString("bookmark", bookmarkClicked);
        extras.putBoolean("addBookmarkClicked", addBookmarkClicked);
        extras.putInt("listViewID", listViewID);
        manageIntent.putExtras(extras);
        startActivity(manageIntent);
    }



    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_add) {
            addBookmarkClicked = true;
            gotoManageActivity(addBookmarkClicked);
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

管理活动

public class ManageActivity extends AppCompatActivity {

    private String title = "EMPTY", url = "EMPTY", note = "EMPTY";
    private boolean listener = true;
    private boolean addBookmarkClicked = false;



/* When activity starts, check for addBookmarkClicked status, create book */
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.manage_layout);

        addBookmarkClicked = getIntent().getExtras().getBoolean("addBookmarkClicked", false);

        String bookmarkString = "";
        bookmarkString = getIntent().getExtras().getString("bookmark");
        if(bookmarkString != null && bookmarkString.length() > 0) {
            if (bookmarkString.length() != 0) {
                String[] stringArray = bookmarkString.split("\\n");
                title = stringArray[0];
                url = stringArray[1];
                note = stringArray[2];
                updateTextViews();
            }
        }
        else { updateTextViews();
        }
    }

    @Override
    public void onBackPressed(){
        Intent bookNoteIntent = new Intent(this, BookNote.class);
        Bundle extras = new Bundle();

        if(title.length() == 0 || title == null){title= "Empty";}
        if(url.length() == 0 || url == null){ url = "Empty";}
        if(note.length() == 0 || url == null) { note = "Empty";}

        extras.putString("title", title);
        extras.putString("url", url);
        extras.putString("note", note);
        extras.putBoolean("listen", listener);
        extras.putBoolean("addBookmarkClicked", addBookmarkClicked);
        bookNoteIntent.putExtras(extras);
        startActivity(bookNoteIntent);
    }

Answer 1

好问题！几周前，我也不得不自己处理这个问题。

主要是您需要重新实例化您的 ArrayAdapter 或 adapter 在您的情况下。

现在你正确地做了第​​一步：

bookmarkList = readData();
adapter = new ArrayAdapter<Bookmark>(this, android.R.layout.simple_list_item_1, bookmarkList);
setListAdapter(adapter);

但是在尝试更新列表时您会再次执行此操作：我相信您正确添加了新书签，但您需要为列表视图重新创建 ArrayAdapter。

编辑： 我相信我发现了危险。 正如我之前提到的，在这里您需要重新实例化 ArrayAdapter。

private void addBookmark(Bookmark bookmark){
    adapter.add(bookmark);
    writeData();
}

基本上，每当您更新要显示在列表中的数据时，您都需要执行此步骤...