好的，我已经制作了常用的Database Helper类，如下所示。我还创建了一个类，它使用getData和insertData方法与另一个类来获取特定的东西，例如使用我的User类来获取用户名。但是，当我想在我的主要活动中调用那个Controller类时，它会告诉我我尝试访问的列不存在。我一直在努力这几个小时，现在累了......
logcat中的表示... sqlite返回：
error code = 1, msg = table userinfo has no column named username
此外，我已将_id和android_metadata等内容添加到我的数据库中。
感谢。
public class DatabaseHelper extends SQLiteOpenHelper{
//The Android's default system path of your application database.
private static final String DB_PATH = "/data/data/com.cslearn/databases/";
private static final String DB_NAME = "example.db";
private static final int DB_VERSION = 1;
private final Context myContext;
private SQLiteDatabase myDatabase;
public DatabaseHelper(Context context) {
super(context, DB_NAME, null, DB_VERSION);
this.myContext = context;
System.out.println(context.getDatabasePath("myDatabase"));
}
public void createDataBase() throws IOException{
System.out.println("database creating...");
boolean dbExist = checkDataBase();
if(dbExist){
//do nothing - database already exist
System.out.println("db exists");
}else{
//By calling this method and empty database will be created into the default system path
//of your application so we are gonna be able to overwrite that database with our database.
this.getReadableDatabase();
System.out.println("path = "+this.getReadableDatabase().getPath());
System.out.println("get database");
try {
this.close();
copyDataBase();
} catch (IOException e) {
throw new Error("Error copying database");
}
}
System.out.println("database created");
this.close();
}
private boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DB_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}catch(SQLiteException e){
//database does't exist yet.
}
if(checkDB != null){
checkDB.close();
}
return checkDB != null ? true : false;
}
private void copyDataBase() throws IOException{
System.out.println("Copying database....");
//Open your local db as the input stream
InputStream myInput = myContext.getAssets().open(DB_NAME);
System.out.println("input > get assets");
// Path to the just created empty db
String outFileName = DB_PATH + DB_NAME;
//Open the empty db as the output stream
OutputStream myOutput = new FileOutputStream(outFileName);
//transfer bytes from the inputfile to the outputfile
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer))>0){
myOutput.write(buffer, 0, length);
System.out.println("output write...");
}
System.out.println("Database copied!!");
//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
/** public void openReadonlyDataBase() throws SQLException{
//Open the database
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}*/
public void openDataBase() throws SQLException{
String myPath = DB_PATH + DB_NAME;
myDatabase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.NO_LOCALIZED_COLLATORS);
}
@Override
public synchronized void close() {
if(myDatabase != null)
myDatabase.close();
super.close();
}
public void insertData (String sql){
try {
this.createDataBase();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
try {
this.openDataBase();
System.out.println("database opened");
}catch(SQLException e){
throw e;
}
myDatabase.execSQL(sql); //separate values with ,
this.close();
}
public ArrayList<String> getData (String table,String [] columns, String selection){
try {
this.createDataBase();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
try {
this.openDataBase();
System.out.println("database opened");
}catch(SQLException e){
throw e;
}
System.out.println("getting data");
ArrayList<String> results = new ArrayList<String>();
Cursor c = myDatabase.query(table, columns, selection, null, null, null, null);
System.out.println(c.getColumnCount());
System.out.println(c.getColumnNames());
System.out.println("got cursor c");
if (c != null) {
/* Check if at least one Result was returned. */
if (c.moveToFirst()) {
do {
/* Retrieve the values of the Entry
* the Cursor is pointing to. */
String[] row = new String[c.getColumnCount()];
for(int i=0; i<c.getColumnCount(); i++){
row[i] = c.getString(i);
System.out.println("getting data");
results.add(row[i]);
System.out.println("adding string");
}
} while (c.moveToNext());
}
}
close();
return results;
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
// Add your public helper methods to access and get content from the database.
// You could return cursors by doing "return myDataBase.query(....)" so it'd be easy
// to you to create adapters for your views.
}
答案 0 :(得分：2)
您已经覆盖了onCreate（SQLiteDatabase db）方法，但它没有做任何事情。您需要在此方法中创建表格，如下所示：
// SQL statements to create new tables.
private static final String TBL_FRIENDS = "friends";
private static final String CREATE_TABLE_FRIENDS = "create table " +
TBL_FRIENDS + " (" + KEY_ID + " integer primary key autoincrement, " +
FRIEND_ID + " integer not null, " + FRIEND_MARKER + " integer not null, " +
FRIEND_MOBILE + " text not null, " + FRIEND_NAME + " text not null);";
@Override
public void onCreate(SQLiteDatabase db)
{
db.execSQL("DROP TABLE IF EXISTS " + TBL_FRIENDS);
db.execSQL(CREATE_TABLE_FRIENDS);
}
显示的其他静态字符串，例如KEY_ID是在其他方法中使用的列名。
答案 1 :(得分：1)
您应该在onCreate()和onUpgrade()中添加一些代码。
onCreate()。在数据库版本增加时调用onUpgrade()。
使用onCreate()和onUpgrade()
onCreate(SQLiteDatabase db)执行创建表的SQL命令。onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)执行删除表的SQL命令（如果存在），然后调用onCreate()（这样表将具有新结构）。当您想要DB_VERSION被呼叫时，不要忘记增加onUpgrade。
答案 2 :(得分：0)
您是否将默认值分配给表的列名？