【发布时间】：2010-01-03 15:36:36
【问题描述】：
我只是在编写一个支持 SQLite 的 Android 应用程序，但是当我调用我的 DatabaseHelper 类时，我不断收到 NullPointerException。似乎导致错误的代码如下：
public Cursor GetAllRows() {
try {
return db.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_PHRASE},
null, null, null, null, null);
} catch (SQLException e) {
Log.e("Exception on query", e.toString());
return null;
}
}
尽管我通常会错过简单的东西，但我已经一遍又一遍地检查代码并没有发现任何错误！
谁能看出有问题？如果您认为错误存在于此，我可以发布更多代码，但我相当确定这是导致错误的块...
更新：数据库适配器的完整源代码。 （如果我没记错的话，这是基于记事本示例）。
包 com.trapp.tts;
导入 android.content.ContentValues; 导入android.content.Context； 导入android.database.Cursor； 导入android.database.SQLException； 导入android.database.sqlite.SQLiteDatabase； 导入 android.database.sqlite.SQLiteOpenHelper； 导入android.util.Log；
公共类 DbAdapter {
public static final String KEY_PHRASE = "phrase";
public static final String KEY_ROWID = "_id";
private static final String TAG = "DbAdapter";
private DatabaseHelper mDbHelper;
private SQLiteDatabase mDb;
/**
* Database creation sql statement
*/
private static final String DATABASE_CREATE =
"create table phrases (_id integer primary key autoincrement, "
+ "phrase text not null);";
private static final String DATABASE_NAME = "db";
private static final String DATABASE_TABLE = " phrases";
private static final int DATABASE_VERSION = 1;
private final Context mCtx;
private static class DatabaseHelper extends SQLiteOpenHelper {
DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(DATABASE_CREATE);
ContentValues cv=new ContentValues();
cv.put(KEY_PHRASE, "1");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "2");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "3");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "4");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "5");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "6");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "7");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "8");
db.insert("phrases", KEY_PHRASE, cv);
cv.put(KEY_PHRASE, "9");
db.insert("phrases", KEY_PHRASE, cv);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
+ newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS notes");
onCreate(db);
}
}
/**
* Constructor - takes the context to allow the database to be
* opened/created
*
* @param ctx the Context within which to work
*/
public DbAdapter(Context ctx) {
this.mCtx = ctx;
}
/**
* Open the notes database. If it cannot be opened, try to create a new
* instance of the database. If it cannot be created, throw an exception to
* signal the failure
*
* @return this (self reference, allowing this to be chained in an
* initialization call)
* @throws SQLException if the database could be neither opened or created
*/
public DbAdapter open() throws SQLException {
mDbHelper = new DatabaseHelper(mCtx);
mDb = mDbHelper.getWritableDatabase();
return this;
}
public void close() {
mDbHelper.close();
}
/**
* Create a new note using the title and body provided. If the note is
* successfully created return the new rowId for that note, otherwise return
* a -1 to indicate failure.
*
* @param title the title of the note
* @param body the body of the note
* @return rowId or -1 if failed
*/
public long createPhrase(String title, String body) {
ContentValues initialValues = new ContentValues();
initialValues.put(KEY_PHRASE, title);
return mDb.insert(DATABASE_TABLE, null, initialValues);
}
/**
* Delete the note with the given rowId
*
* @param rowId id of note to delete
* @return true if deleted, false otherwise
*/
public boolean deletePhrase(long rowId) {
return mDb.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}
/**
* Return a Cursor over the list of all notes in the database
*
* @return Cursor over all notes
*/
public Cursor fetchAllPhrases() {
return mDb.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_PHRASE},
null, null, null, null, null);
}
/**
* Return a Cursor positioned at the note that matches the given rowId
*
* @param rowId id of note to retrieve
* @return Cursor positioned to matching note, if found
* @throws SQLException if note could not be found/retrieved
*/
public Cursor fetchPhrase(long rowId) throws SQLException {
Cursor mCursor =
mDb.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
KEY_PHRASE}, KEY_ROWID + "=" + rowId, null,
null, null, null, null);
if (mCursor != null) {
mCursor.moveToFirst();
}
return mCursor;
}
/**
* Update the note using the details provided. The note to be updated is
* specified using the rowId, and it is altered to use the title and body
* values passed in
*
* @param rowId id of note to update
* @param title value to set note title to
* @param body value to set note body to
* @return true if the note was successfully updated, false otherwise
*/
public boolean updatePhrase(long rowId, String title, String body) {
ContentValues args = new ContentValues();
args.put(KEY_PHRASE, title);
return mDb.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
}
}
调用代码：
private void fillData() {
mCursor = dbHelper.fetchAllPhrases();
startManagingCursor(mCursor);
ListAdapter adapter = new SimpleCursorAdapter(
this,
android.R.layout.simple_list_item_1,
mCursor,
new String[] {"phrase"},
new int[] {}
);
setListAdapter(adapter);
}
调用 .fetchAllPhrases() 时似乎崩溃了
mCursor = dbHelper.fetchAllPhrases();
【问题讨论】：
-
你试过设置断点调试看看
db是否为空吗？