Select语句不适用于sqlite android

我是android开发新手。我想要的是从数据库中存储的值填充我的列表。因此，我使用insert和select语句创建了一个数据库。我还创建了一个模型来保存数据。我从列表适配器将数据插入数据库，但当我试图检索该数据时，我得到了null

数据库类

public class DbHandler extends SQLiteOpenHelper {

    private static final int dbVersion = 1;
    private static final String dbName = "HSsuraksha";
    private static final String tableName = "pocketDocs";
    private static final String userId = "userId";
    private static final String docId = "docId";
    private static final String fileName = "fileName";
    private static final String fileExt = "fileExt";
    private static final String title = "title";

    private static final String createTable = "CREATE TABLE " + tableName + "(" + userId + " Integer Primary Key," + docId + " Integer," + fileName + " Text," + fileExt + " Text," + title + " Text" + ")";

    public DbHandler(Context context) {
        super(context, dbName, null, dbVersion);
    }

    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
        sqLiteDatabase.execSQL(createTable);
    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i2) {

    }

    public void insertData(DbModel dbModel) {
        SQLiteDatabase database = getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        database.beginTransaction();
        contentValues.put(userId, dbModel.userId);
        contentValues.put(docId, dbModel.docId);
        contentValues.put(fileName, dbModel.fileName);
        contentValues.put(fileExt, dbModel.fileExtension);
        contentValues.put(title, dbModel.title);
        if (contentValues != null) {
            Long id = database.insert(tableName, null, contentValues);
            Log.e("insert values", "" + id);
        }

        database.endTransaction();
        database.close();

    }

    public DbModel selectDocs(String id) {
        SQLiteDatabase db = this.getReadableDatabase();
        DbModel model = new DbModel();
        Cursor cursor = db.query(tableName, new String[]{fileName, fileExt, title}, docId + "=?", new String[]{id}, null, null, null, null);
        if (cursor.moveToFirst()) {
            model.fileName = cursor.getString(2);
            model.fileExtension = cursor.getString(3);
            model.title = cursor.getString(4);
        }
        db.close();
        return model;
    }
}

数据库模型

public class DbModel implements Serializable {
    public String userId, docId, fileName, fileExtension, title;

    public DbModel() {
    }

    public DbModel(String userId, String docId, String fileName, String fileExtension, String title) {
        this.userId = userId;
        this.docId = docId;
        this.fileName = fileName;
        this.fileExtension = fileExtension;
        this.title = title;
    }

    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    public String getDocId() {
        return docId;
    }

    public void setDocId(String docId) {
        this.docId = docId;
    }

    public String getFileName() {
        return fileName;
    }

    public void setFileName(String fileName) {
        this.fileName = fileName;
    }

    public String getFileExtension() {
        return fileExtension;
    }

    public void setFileExtension(String fileExtension) {
        this.fileExtension = fileExtension;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

列表适配器

public View getView(int i, View view, ViewGroup viewGroup) {
        final ViewHolder holder;
        dbHandler = new DbHandler(context);
        dbModel = new DbModel("1", "1", "GoogleDocs", ".pdf", "Test");

        dbHandler.insertData(dbModel);
        if (view == null) {
            holder = new ViewHolder();
            view = LayoutInflater.from(context).inflate(R.layout.view_docs_custom_list, viewGroup, false);
            holder.docsImage = (ImageView) view.findViewById(R.id.iv_docs_image);
            holder.tvTitle = (TextView) view.findViewById(R.id.tv_file_name);
            holder.tvPostedOn = (TextView) view.findViewById(R.id.tv_posted_on);
            holder.btShare = (Button) view.findViewById(R.id.bt_share);
            view.setTag(holder);
        } else {
            holder = (ViewHolder) view.getTag();
        }


        dbHandler.selectDocs(dbModel.getDocId());    // here i am getting the values from db and displaying it in the list

        holder.tvTitle.setText(dbModel.title);

改变

到

getString

基于零索引。它取决于您在

query

中提到的列数，而不是在

CREATE

语句中使用它们的顺序

如果你想做

cursor.getString(3);

您应该得到一个

非法状态异常

声明

java.lang.IllegalStateException: Couldn't read row 0, col 3 from CursorWindow.  Make sure the Cursor is initialized correctly before accessing data from it.

---

因为您试图访问

光标中的第4列，而它只有3列
dbHandler.selectDocs（dbModel.getDocId（））返回模型对象。你是说这个
dbModel.getDocId（）
失败？？在select方法“dbModel model=new dbModel（）；”中，model retuns null你确定吗？我不认为
model
为null。您是否获得了
NullPointerException
？如果是这样发布stacktraceno，我不会得到null指针异常，但当我调试它时，我会在调试应用程序时，在这行“If（cursor.moveToFirst（））”之后，将DbModel中的所有值都作为null，它不会进入If backet，当数据库中没有数据时，directlt就会出现。您需要调用
database.setTransactionSuccessful（）
在
数据库.endTransaction（）之前在您的
insertData
方法中，感谢它的工作nw，但在“model.fileExtension=cursor.getString（3）；”行中，我得到了错误
cursor.getString(3);

java.lang.IllegalStateException: Couldn't read row 0, col 3 from CursorWindow.  Make sure the Cursor is initialized correctly before accessing data from it.