我正在为学校分配工作，说明是创建一个包含两个活动的书签应用程序，一个名为BookNote，必须是List活动，另一个名为ManageActivity。
BookNote是在启动时使用从文件读取的数据填充List活动，如果没有文件，则创建示例书签。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，它应该改变列表。
我现在的代码有评论显示我对每个函数的意图，但我当前的问题是每次从ManageActivity返回BookNote时，应用程序崩溃时出现错误“尝试调用虚方法”void android.widget.ArrayAdapter尝试将新书签对象添加或插入ArrayAdapter时，.insert（java.lang.Object，int）'对空对象引用'。
BookNote活动
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);
}
}
ManageActivity
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();
}
}
答案 0 :(得分：2)
您正在声明适配器的全局和本地实例。在BookNote活动的onCreate()方法中，删除本地实例。变化
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);
答案 1 :(得分：1)
您没有设置包含
的xml文件book_layout.xml
<ListView android:id="@+id/android:list"
android:layout_width="fill_parent"
android:layout_height="fill_parent"/>
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.book_layout);
ArrayList<Bookmark> bookmarkList = readData();
ArrayAdapter adapter = new ArrayAdapter<Bookmark>(this, android.R.layout.simple_list_item_1, bookmarkList);
setListAdapter(adapter);