在Android的SQLite中创建数据库时出现问题

Question

Hi I am new to android and I have a problem in creating a database.

public class database extends ListActivity {
    /** Called when the activity is first created. */
    private final String MY_DATABASE_NAME = "myCoolUserDB.db";
    private final String MY_DATABASE_TABLE = "t_Users"; 
    Context c;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ArrayList<String> results = new ArrayList<String>();
        setContentView(R.layout.main);
        SQLiteDatabase mydb=null;
        try
        {
            mydb.openOrCreateDatabase(MY_DATABASE_NAME,  null);

        } catch(Exception e){}
    }
}

When I run this code it throws a run time exception. Please help me.

Answer 1

1. If you are going to call a static method like openOrCreateDatabase, do it on the class (SQLiteDatabase.openOrCreateDatabase(...)), not an instance. It's a lot clearer - the way you've done it looks like you're calling an instance method, so looks like a sure NullPointerException, which of course is misleading.

2. As someone else has stated, the stack trace would be the most useful thing when asking for help with an exception.

3. (Almost) never catch an exception without at the very least logging it. Don't just do nothing with it. There are of course exceptions to every rule, but let's not go there for the moment. Anyway, if you don't at least log it, you're just throwing away information that would tell you what went wrong when everything goes to crap later.

4. You shouldn't be using that method directly, and should instead be extending SQLiteOpenHelper . See the android developers page on data storage to get started (I'd post a link but apparently I'm only allowed one link in my post ?!), and since you've probably had to download the SDK to get going, look in the samples that come with it for the Notepad sample application. That contains a NotePadProvider class, which is a good example of both a content provider and database access, which often go hand-in-hand on android. I'd suggest compiling that application and making some simple changes to it before you jump into making your own one.

Answer 2

For working with sqlite database you need to create class extended from SQLiteOpenHelper:

private class DBHelper extends SQLiteOpenHelper {   

    public DBHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_TABLES);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL(UPGRADE_TABLES);
    }

}   

Then you can get access to db using DbHelper object:

DBHelper dbHelper = new DBHelper(Activity.this);
SQLiteDatabase db = dbHelper.getReadableDatabase();

Answer 3

I run into the same problem. It figures out that two bugs happens during development

• dir "databases" was not existent
• accendently ".db" was created as directory.

They following code cover both

File dbFile = getDatabasePath ("abc.db");

if (dbFile.isDirectory ()) {
    dbFile.delete();
}
if (! dbFile.exists()) {
    String path = dbFile.getParent ();
    new File (path).mkdirs ();
}
database  = SQLiteDatabase.openDatabase (dbFile.getAbsolutePath (), this, SQLiteDatabase.OPEN_READWRITE | SQLiteDatabase.CREATE_IF_NECESSARY);

Hope this helps

I think SQLiteOpenHelper is only useful for "single table" databases. For multiple table applications I consider directly using SQLiteDatabase fit better to a good architecture.

Answer 4

这是一篇简单的文章，告诉您如何在Android的SQLite数据库中插入数据 ，此外，此链接还向您展示了如何从Android的SQLite数据库检索数据 。