我对此感到困惑。我正在关注书中的一个项目，如果我使用书籍示例文件，它就可以工作。但是，如果我使用我的文件，据我所知是相同的，我得到一个空指针异常。这是下面的文件：
//AddressBook.java
//Main activity for address book app.
package com.deitel.addressbook;
import android.app.ListActivity;
import android.os.Bundle;
import android.os.AsyncTask;
import android.database.Cursor;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.MenuItem;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.CursorAdapter;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.util.Log;
public class AddressBook extends ListActivity {
//string used when logging for debug
public static final String TAG ="AddressBookBot";
public static final String ROW_ID = "row_id"; //Intent extra key
private ListView contactListView; //the ListActivity's ListView
private CursorAdapter contactAdapter; //adapter for ListView
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState); //call super's onCreate
Log.d(TAG, "In onCreate() event");
contactListView = getListView();
contactListView.setOnItemClickListener(viewContactListener);
//map each contact's name to a TV in the ListView layout
String[] from = {"name"};
int[] to = new int[] {R.id.contactTextView};
CursorAdapter contactAdapter = new SimpleCursorAdapter(AddressBook.this,
R.layout.contact_list_item, null, from, to);
setListAdapter(contactAdapter); //set contactView's adapter
}
@Override
protected void onResume(){
Log.d(TAG, "In onResume() event");
super.onResume(); //call super's onResume method
//create a new GetContactsTask and execute it
new GetContactsTask().execute((Object[]) null);
} //end onResume
@Override
protected void onStop(){
Log.d(TAG, "In onStop() event");
Cursor cursor = contactAdapter.getCursor(); //get current Cursor
if (cursor != null)
cursor.deactivate(); //deactivate it
contactAdapter.changeCursor(null); //adapter now has no Cursor
super.onStop();
} //end onStop
//performs database query outside GUI thread
private class GetContactsTask extends AsyncTask<Object, Object, Cursor>{
DatabaseConnector DBC = new DatabaseConnector(AddressBook.this);
//perform the database access
@Override
protected Cursor doInBackground(Object... params){
DBC.open();
//get a cursor containing call contacts
return DBC.getAllContacts();
} //end method doInBackground
//use the Cursor returned from the doInBackground method
@Override
protected void onPostExecute(Cursor result){
Log.d(TAG, "In onPostExecute() event");
contactAdapter.changeCursor(result); //set the adapter's Cursor
DBC.close();
} //end method onPostExecute
} //end class GetContactsTask
//create the Activity's menu from a menu resource XML file
public boolean onCreateOptionsMenu(Menu menu) {
super.onCreateOptionsMenu(menu);
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.addressbook_menu, menu);
return true;
} //end onCreateOptionsMenu
//handle the choice from options menu
@Override
public boolean onOptionsItemSelected(MenuItem item){
//create a new Intent to launch the AddEditContact Activity
Intent addNewContact = new Intent(AddressBook.this, AddEditContact.class);
startActivity(addNewContact); //start the AddEditContact Activity
return super.onOptionsItemSelected(item); //call super's method
} //end onOptionsItemSelected
//event listener that responds to the users touching a contact's name in the ListView
OnItemClickListener viewContactListener = new OnItemClickListener(){
@Override
public void onItemClick(AdapterView<?> arg0, View arg1, int arg2, long arg3){
//create an Intent to launch the ViewContact Activity
Intent viewContact = new Intent(AddressBook.this, ViewContact.class);
//pass the selected contact's row ID as an extra with the intent
viewContact.putExtra(ROW_ID, arg3);
startActivity(viewContact); //start the ViewContact Activity
} //end method onItemClick
}; //end viewContactListener
} //end class AddressBook
问题似乎是我的“getContactsTask”内部类中的这一行：contactAdapter.changeCursor(result); 一旦我点击这条线，我就会得到一个空指针异常。作为参考，这里是我在 getContactsTask 中引用的数据库连接器类：
//DatabaseConnector.java
//Provides easy connection and creation of UserContacts database.
package com.deitel.addressbook;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.util.Log;
public class DatabaseConnector {
public static final String TAG ="AddressBookBot";
//database name
private static final String DATABASE_NAME = "UserContacts";
private SQLiteDatabase database; //database object
private DatabaseOpenHelper DBOpenHelper; //database helper
//public constructor for DatabaseConnector
public DatabaseConnector(Context context){
//create a new DatabaseOpenHelper
DBOpenHelper = new DatabaseOpenHelper(context, DATABASE_NAME, null, 1);
} //end constructor
//open the database connection
public void open() throws SQLException{
//create or open a database for reading/writing
database = DBOpenHelper.getWritableDatabase();
} //end method open
//close the database connection
public void close(){
if(database != null)
database.close(); //close database connection
} //end method close
//inserts a new contact in the database
public void insertContact(String name, String email, String phone, String state, String city){
ContentValues newContact = new ContentValues();
newContact.put("name", name);
newContact.put("email", email);
newContact.put("phone", phone);
newContact.put("street", state);
newContact.put("city", city);
open(); //open the database
database.insert("contacts", null, newContact);
close(); //close the database
} //end insertContact
//inserts a new contact in the database
public void updateContact(long id, String name, String email, String phone, String state, String city){
ContentValues editContact = new ContentValues();
editContact.put("name", name);
editContact.put("email", email);
editContact.put("phone", phone);
editContact.put("street", state);
editContact.put("city", city);
open(); //open the database
database.update("contacts", editContact, "_id=" + id, null);
close(); //close the database
} //end updateContact
//return a Cursor with all contact information in the database
public Cursor getAllContacts(){
Log.d(TAG, "in getAllContacts");
return database.query("contacts", new String[] {"_id", "name"}, null, null, null, null, "name");
} //end getAllContacts
//get a Cursor containing all information about the contact specified by the given id
public Cursor getOneContact(long id){
return database.query("contacts", null, "_id='" + id, null, null, null, null);
} //end getOneContact
//delete the contact specified by the given String name
public void deleteContact(long id){
open(); //open the database
database.delete("contacts", "_id=" + id, null);
close(); //close the database
} //end deleteContact
private class DatabaseOpenHelper extends SQLiteOpenHelper{
//public constructor
public DatabaseOpenHelper(Context context, String name, CursorFactory factory, int version){
super(context, name, factory, version);
} //end DatabaseOpenHelper contstructor
//creates the contacts table when the database is created
@Override
public void onCreate(SQLiteDatabase db){
//query to create a new table named contacts
String createQuery = "CREATE TABLE contacts" + "(_id integer primary key autoincrement," + "name TEXT, email TEXT, phone TEXT," + "street TEXT, city TEXT);";
db.execSQL(createQuery); //execute the query
} //end method onCreate
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
} //end onUpgrade
} //end class DatabaseOpenHelper
} //end class DataBaseConnector
对此的任何帮助或澄清将不胜感激！