java - 尝试添加到 ArrayAdapter 时应用程序崩溃

Question

我正在为学校做一项作业，说明是创建一个包含两个 Activity 的书签应用程序，一个名为 BookNote，必须是列表 Activity ，另一个名为 ManageActivity。

BookNote 将在启动时使用从文件读取的数据填充列表 Activity ，如果不存在文件，则会创建示例书签。可以添加、编辑和删除书签，但所有输入都必须在 ManageActivity 中完成。每当 Booknote 处理从 ManageActivity 返回的数据时，它都应该更改列表。

我现在的代码有注释显示我对每个函数的 Intent ，但我当前的问题是每次我从 ManageActivity 返回 BookNote 时，应用程序都会崩溃并显示错误“尝试调用虚拟方法 'void android.widget.ArrayAdapter尝试向 ArrayAdapter 添加或插入新的书签对象时，在空对象引用上插入 (java.lang.Object, int)'。

BookNote Activity

import android.app.ListActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;


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 boolean addBookmarkClicked = false;



    /*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);


        ArrayList<Bookmark> bookmarkList = readData();
        ArrayAdapter 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().getBooleanExtra("addClicked", 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();

            if(addBookmarkClicked == true) {
                addBookmark(bookmark);
            }
            else {
                insertBookmark(bookmark, listViewID);
            }
            //writeData();
        }
    }

    //When list item is clicked, fill bookmarkClicked with values in list item, record item position, and call gotoManageActivity() to pass information to ManageActivity.

    public void onListItemClick(ListView listView, View view, int position, long id) {
        ListView myListView = getListView();
        Object itemClicked = myListView.getAdapter().getItem(position);
        Toast.makeText(getApplicationContext(), "Item clicked: " + itemClicked + "\nItem ID: " + id,Toast.LENGTH_SHORT).show();
        bookmarkClicked = itemClicked.toString();
        listViewID = position;
        gotoManageActivity();
    }


    //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;
    }


    //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();
    }

    //If a list item is clicked, any edits will be inserted back into the  ArrayAdapter in the same position of the item clicked.
    private void insertBookmark(Bookmark bookmark, int listViewID){
        adapter.insert(bookmark,listViewID);
        //writeData();
    }


    //Calls ManageActivity and reports information about app.
    public void gotoManageActivity(){
        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 onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is   present.
        getMenuInflater().inflate(R.menu.menu_book_note, menu);
        return true;
    }

    @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();
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

管理 Activity

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.text.InputType;
import android.view.KeyEvent;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;

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().getBooleanExtra("addClicked", 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);
        startActivity(bookNoteIntent);
        Bundle extras = new Bundle();
        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);
    }


    public void editButtonCall(View view){
        showNoteDialog();
        showURLDialog();
        showTitleDialog();
    }

    public void webButton(View view){
        if(url.length() != 0 && url != "EMPTY"){

        }
    }

    public void updateTextViews(){
        TextView titleTextView = (TextView) findViewById(R.id.titleTV);
        TextView urlTextView = (TextView) findViewById(R.id.urlTV);
        TextView noteTextView = (TextView) findViewById(R.id.noteTV);
        titleTextView.setText("Title: " + title);
        urlTextView.setText("URL: " + url);
        noteTextView.setText("Note: " + note);
    }

    //Show dialog and editText for user to assign title to bookmark.
    public void showTitleDialog(){
        AlertDialog.Builder titleBuilder = new AlertDialog.Builder(this);
        titleBuilder.setTitle("Enter Title");

        final EditText titleET = new EditText(this);
        titleET.setInputType(InputType.TYPE_TEXT_VARIATION_NORMAL);
        titleBuilder.setView(titleET);
        titleBuilder.setPositiveButton("Add", newDialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int i) {
                title = titleET.getText().toString();
                updateTextViews();
            }
        });
        titleBuilder.show();
    }


    //Show dialog and editText for user to assign note to bookmark.
    public void showNoteDialog(){
        AlertDialog.Builder noteBuilder = new AlertDialog.Builder(this);
        noteBuilder.setTitle("Enter Note");

        final EditText noteET = new EditText(this);
        noteET.setInputType(InputType.TYPE_TEXT_VARIATION_NORMAL);
        noteBuilder.setView(noteET);

        noteBuilder.setPositiveButton("Add", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                note = noteET.getText().toString();
                updateTextViews();
            }
        });

        noteBuilder.show();
    }

    //Show dialog and editText for user to assign url to bookmark.
    public void showURLDialog(){
        AlertDialog.Builder urlBuilder = new AlertDialog.Builder(this);
       urlBuilder.setTitle("Enter URL");

        final EditText urlET = new EditText(this);
        urlET.setInputType(InputType.TYPE_TEXT_VARIATION_NORMAL);
        urlBuilder.setView(urlET);

        urlBuilder.setPositiveButton("Add", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int i) {
                url = urlET.getText().toString();
                updateTextViews();
            }
        });
        urlBuilder.show();
    }
}

Answer 1

您正在声明适配器的全局实例和本地实例。在BookNote Activity的onCreate()方法中，删除本地实例。改变

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

到

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