在Android上将新记录插入SQLite时出现问题

行“return db.insert（DATABASE_TABLE，null，initialValues）；”返回一个null指针，我不明白为什么！如果有任何帮助，我将不胜感激

public class Database extends Activity {

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);        
    DBAdapter db = new DBAdapter(this);
    if(db != null){
     long id = db.insertUser("test", "test");
     Cursor c = db.getUser(id);        
     Log.d("DB", c.getString(1));
     Log.d("DB", c.getString(2));
    }
    db.close();
}
}

-

---

在插入之前必须调用Getwriteabledatabase。我在open（）中看到了它，但我没有看到open在哪里被调用。这是我对问题的最佳猜测。

另外，

onCreate

可以在null

DBAdapter

上调用

close（）

。

public class DBAdapter 
{
    public static final String KEY_ROWID = "_id";
    public static final String KEY_USERNAME = "username";
    public static final String KEY_PASSWORD = "password";
    private static final String TAG = "DBAdapter";

    private static final String DATABASE_NAME = "test";
    private static final String DATABASE_TABLE = "users";
    private static final int DATABASE_VERSION = 1;

    private static final String DATABASE_CREATE =
        "create table IF NOT EXISTS users (_id integer primary key autoincrement, "
        + "username text, password text);";

    private final Context context; 

    private DatabaseHelper DBHelper;
    private SQLiteDatabase db;

    public DBAdapter(Context ctx) 
    {
        this.context = ctx;
        DBHelper = new DatabaseHelper(context);
    }

    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);
        }

        @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 titles");
            onCreate(db);
        }
    }    

    //---opens the database---
    public DBAdapter open() throws SQLException 
    {
        db = DBHelper.getWritableDatabase();
        return this;
    }

    //---closes the database---    
    public void close() 
    {
        DBHelper.close();
    }

    //---insert a title into the database---
    public long insertUser(String username, String password) 
    {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_USERNAME, username);
        initialValues.put(KEY_PASSWORD, password);
        return db.insert(DATABASE_TABLE, null, initialValues);
    }

    //---deletes a particular title---
    public boolean deleteTitle(long rowId) 
    {
        return db.delete(DATABASE_TABLE, KEY_ROWID + 
          "=" + rowId, null) > 0;
    }

    //---retrieves all the titles---
    public Cursor getAllUsers() 
    {
        return db.query(DATABASE_TABLE, new String[] {
          KEY_ROWID, 
          KEY_USERNAME,
          KEY_PASSWORD,
               }, 
                null, 
                null, 
                null, 
                null, 
                null);
    }

    //---retrieves a particular title---
    public Cursor getUser(long rowId) throws SQLException 
    {
        Cursor mCursor =
                db.query(true, DATABASE_TABLE, new String[] {
                  KEY_ROWID,
                  KEY_USERNAME, 
                  KEY_PASSWORD,                  
                  }, 
                  KEY_ROWID + "=" + rowId, 
                  null,
                  null, 
                  null, 
                  null, 
                  null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    //---updates a title---
    public boolean updateTitle(long rowId, String username, 
    String password) 
    {
        ContentValues args = new ContentValues();
        args.put(KEY_USERNAME, username);
        args.put(KEY_PASSWORD, password);

        return db.update(DATABASE_TABLE, args, 
                         KEY_ROWID + "=" + rowId, null) > 0;
    }
}