即使增加数据库版本也不会调用 onCreate()

Question

我正在尝试使用 onUpgrade() 添加新的 table。我已经给出了在其中创建一个 table 的查询，如果旧版本小于新版本，则执行该查询。我已经增加了数据库版本，但我在 onUpgrade() 中的 db.execSQL() 中得到了 NullPointerException for db。

我在调试模式下查看并看到 onUpgrade() 中的新版本（我代码中的 i2）仍然是 7（见代码），尽管我已更改为 8。

为什么我在 onUpgrade() 中 db 得到 NullPointerException。我已将 onCreate() 中的 SQLiteDatabase db 的值定义为 this.db = db（我从方法 onCreate(SQLiteDatabase db,..) 中获得的值）并且我在构造函数中调用了 onUpgrade class 扩展 SQLiteOpenHelper 并将 oldVersion 和 newVersion 值传递给它。抱歉，如果我试图解释太多而让您感到困惑。我是 android 的新手，不知道这是否是向现有数据库添加新 table 的正确方法。

代码如下：

DatabaseHelper.class

public class DatabaseHelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION = 9;
private static final String DATABASE_NAME = "database";
SQLiteDatabase db;
ArrayList<String> arrayList;
Context cont;

DatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    this.cont = context;
    arrayList = new ArrayList<>();

    db = this.getWritableDatabase();
    this.onUpgrade(db, 7, 8);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("create table tableName(name text,email text primary key,password varchar,phno int)");
    db.execSQL("create table bookingDetails(email text, date text, time text,vehicle text,fromLocation text,destination text)");
    db.execSQL("create table emptyTable (name text)");
    db.execSQL("insert into emptyTable values(ad)");
    this.db = db;
}

public boolean save(String name, String email, String password, String phno) {
    db = this.getWritableDatabase();
    ContentValues values = new ContentValues();
    values.put("name", name);
    values.put("email", email);
    values.put("password", password);
    values.put("phno", phno);
    long rows;
    rows = db.insert("tableName", null, values);
    if (rows <= 0) {
        Toast.makeText(cont, "Failed", Toast.LENGTH_LONG).show();
        return false;

    }
    return true;
}

public String getUserName(String email) {
    Cursor c = db.rawQuery("select name from tableName where email = '" + email + "'", null);
    c.moveToFirst();
    return c.getString(0);

}

public void saveBookingDetails(String email, String date, String time, String vehicleType, String fromLocation, String destination) {
    db = this.getWritableDatabase();
    ContentValues values = new ContentValues();
    values.put("email", email);
    values.put("date", date);
    values.put("time", time);
    values.put("vehicle", vehicleType);
    values.put("fromLocation", fromLocation);
    values.put("destination", destination);
    long rows;
    rows = db.insert("bookingDetails", null, values);
    if (rows <= 0) {
        Toast.makeText(cont, "Failed", Toast.LENGTH_LONG).show();
    } else
        Toast.makeText(cont, "Saved", Toast.LENGTH_LONG).show();
}


public void delete() {
    db.execSQL("delete from bookingDetails");
}

public ArrayList displayUserDetails(String email) {
    int i = 0;
    Cursor cursor = db.rawQuery("select name,email,phno from tableName where email = '" + email + "'", null);
    cursor.moveToFirst();
    do {
        arrayList.add(cursor.getString(cursor.getColumnIndex("name")));
        arrayList.add(cursor.getString(cursor.getColumnIndex("email")));
        arrayList.add(cursor.getString(cursor.getColumnIndex("phno")));
        i++;
    } while (cursor.moveToNext());
    return arrayList;
}

public boolean getLogin(String email, String pass) {
    db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery("select password from tableName where email = '" + email + "'", null);
    if (cursor.moveToFirst() && cursor.getCount() > 0 && cursor != null) {
        if (cursor.getString(0).equals((pass))) {
            return true;
        }
    }
    return false;

}

public long getTaskCount() {
    return DatabaseUtils.queryNumEntries(getReadableDatabase(), "tableName");
}

public ArrayList getBookingHistory(String email) {
    ArrayList bookingHistoryList = new ArrayList();
    String e = email;
    Cursor c = db.rawQuery("select * from bookingDetails where email = '" + email + "'", null);
    int cou = c.getCount();
    c.moveToFirst();
    do {
        bookingHistoryList.add(c.getString(c.getColumnIndex("date")));
        bookingHistoryList.add(c.getString(c.getColumnIndex("time")));
        bookingHistoryList.add(c.getString(c.getColumnIndex("fromLocation")));
        bookingHistoryList.add(c.getString(c.getColumnIndex("destination")));
        bookingHistoryList.add(c.getString(c.getColumnIndex("vehicle")));
    }
    while (c.moveToNext());
    return bookingHistoryList;
}

public ArrayList getTextViewText() {
    ArrayList txtViewList = new ArrayList();
    Cursor cursor = db.rawQuery("select * from tableName", null);
    cursor.moveToFirst();
    do {
        txtViewList.add(cursor.getString(cursor.getColumnIndex("name")));
        txtViewList.add(cursor.getString(cursor.getColumnIndex("email")));
        txtViewList.add(cursor.getString(cursor.getColumnIndex("phno")));
    } while (cursor.moveToNext());

    return txtViewList;
}

public void saveOnEdit(String email, String name, String phno, String password) {
    ContentValues values = new ContentValues();


    if (name.length() == 0) {
        if (phno.length() == 0) {
            values.put("password", password);
            db.update("tableName", values, "email = '" + email + "'", null);
        } else if (password.length() == 0) {
            values.put("phno", phno);
            db.update("tableName", values, "email = '" + email + "'", null);
        } else {
            values.put("phno", phno);
            values.put("password", password);
        }
    } else if (password.length() == 0 && phno.length() == 0) {
        values.put("name", name);
        db.update("tableName", values, "email = '" + email + "'", null);
    } else if (password.length() == 0) {
        values.put("name", name);
        values.put("phno", phno);
        db.update("tableName", values, "email = '" + email + "'", null);
    } else if (phno.length() == 0) {
        values.put("name", name);
        values.put("password", password);
        db.update("tableName", values, "email = '" + email + "'", null);
    } else {
        values.put("name", name);
        values.put("password", password);
        values.put("phno", phno);
        db.update("tableName", values, "email = '" + email + "'", null);
    }
}

public String empty() {
    Cursor c = db.rawQuery("select name from emptyTable", null);
    return c.getString(c.getColumnIndex("name"));
}

@Override
public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i2) {
    if (i < i2) {
        db.execSQL("create table emptyTable(name text)");
        ContentValues values = new ContentValues();
        values.put("name", "ad");
        db.insert("emptyTable", null, values);
    }

}

@Override
public void onConfigure(SQLiteDatabase db) {
    super.onConfigure(db);
}

@Override
public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    super.onDowngrade(db, oldVersion, newVersion);
}

@Override
public void onOpen(SQLiteDatabase db) {
    super.onOpen(db);
}
}

Answer 1

在 onUpgrade 中，您使用了错误的参考：

    db.execSQL("create table emptyTable(name text)");

而不是

    sqLiteDatabase.execSQL("create table emptyTable(name text)");

您的构造函数将调用升级并在您获得数据库引用之前创建，因此升级时它仍然为空。

Answer 2

当数据库版本增加时，您必须通过在 onUpgrade 中调用它来调用 onCreate，如下所示：

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    onCreate(db);
}

即使增加数据库版本也不会调用 onCreate()

onCreate() is not called even when Database version is increased

android

android-sqlite