將數據庫另存為CSV文件

Question

我是新來的。 當用戶通過測試時，我得到了具有4項考試的應用程序。 用戶名和考試日期以及跟蹤代碼，答案將保存在數據庫中。 另一方面，用戶可以看到檢查結果，該結果已經保存在數據庫中，並且我有一個列表視圖，該視圖顯示數據庫（SQlite數據庫）中的數據，所以我得到了一個Button，當用戶單擊它時，應該將數據庫另存為CSV文件。 我已經在項目中導入了OPENCSV庫。 所以我在我的按鈕上寫代碼。 但是在運行時，當我單擊按鈕時。 什么也沒發生。 並在logcat中出現錯誤：

05-29 21：17：43.198 2531-2531 / ir.arbn.www.mytestprogramsamanik E / ResultActivity：嘗試調用虛擬方法'android.database.sqlite.SQLiteDatabase ir.arbn.www.mytestprogramsamanik.DatabaseHelper.getReadableDatabase（）'空對象引用上的java.lang.NullPointerException：嘗試在空對象引用上調用虛擬方法'android.database.sqlite.SQLiteDatabase ir.arbn.www.mytestprogramsamanik.DatabaseHelper.getReadableDatabase（）'

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數據庫類：

public class DatabaseHelper extends SQLiteOpenHelper {
public static final String DATABASE_NAME = "person.db";
public static final int VERSION = 1;
public static final String TABLE_NAME = "tbl_person";
public static final String S_ID = "id";
public static final String S_NAME = "name";
public static final String S_DATE = "date";
public static final String S_TRACK = "track";
public static final String S_ANSWER = "answer";
public static final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + "" + "(" + S_ID + " INTEGER PRIMARY KEY AUTOINCREMENT," + S_NAME + " TEXT NOT NULL," + S_DATE + " TEXT NOT NULL," + S_TRACK + " TEXT NOT NULL," + S_ANSWER + " TEXT NOT NULL)";

public DatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, VERSION);
}

public void insertPerson(String name, String date, String track, String answer) {
    ContentValues contentValues = new ContentValues();
    contentValues.put(S_NAME, name);
    contentValues.put(S_DATE, date);
    contentValues.put(S_TRACK, track);
    contentValues.put(S_ANSWER, answer);
    SQLiteDatabase db = this.getWritableDatabase();
    db.insert(TABLE_NAME, null, contentValues);
    db.close();
}

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

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)      {
    db.execSQL("DROP TABLE" + TABLE_NAME);
    onCreate(db);
}

public ArrayList<PersonModel> getAllPersonData() {
    ArrayList<PersonModel> list = new ArrayList<>();
    String sql = "SELECT * FROM " + TABLE_NAME;
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(sql, null);
    if (cursor.moveToFirst()) {
        do {
            PersonModel pm = new PersonModel();
            pm.setId(cursor.getInt(0) + "");
            pm.setName(cursor.getString(1));
            pm.setDate(cursor.getString(2));
            pm.setTrack(cursor.getString(3));
            pm.setAnswer(cursor.getString(4));
            list.add(pm);
        } while (cursor.moveToNext());
    }
    return list;
}
}

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即使在這里，您也可以看到帶有Listview和保存按鈕的Activity：

public class ResultActivity extends AppCompatActivity {
@BindView(R.id.listView)
ListView listView;
@BindView(R.id.save)
CustomButton save;
CustomAdapter customAdapter;
ArrayList<PersonModel> arrayList;
DatabaseHelper db;
DatabaseHelper dbhelper;
Context mContext = this;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_result);
    ButterKnife.bind(this);
    db = new DatabaseHelper(this);
    arrayList = new ArrayList<>();
    arrayList = db.getAllPersonData();
    customAdapter = new CustomAdapter(this, arrayList);
    listView.setAdapter(customAdapter);
    listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
            Intent intent = new Intent(mContext, FragmentActivity.class);
            startActivity(intent);
        }
    });
    save.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
           DatabaseHelper db = new DatabaseHelper(getApplicationContext());
            File exportDir = new File(Environment.getExternalStorageDirectory(), "");
            if (!exportDir.exists()) {
                exportDir.mkdirs();
            }
            File file = new File(exportDir, "csvname.csv");
            try {
                file.createNewFile();
                    CSVWriter csvWrite = new CSVWriter(new FileWriter(file));
                SQLiteDatabase myDb = dbhelper.getReadableDatabase();
                Cursor curCSV = myDb.rawQuery("SELECT * FROM tbl_person", null);
                csvWrite.writeNext(curCSV.getColumnNames());
                while (curCSV.moveToNext()) {
                    String arrStr[] = {curCSV.getString(0), curCSV.getString(1), curCSV.getString(2)};
                    csvWrite.writeNext(arrStr);
                }
                csvWrite.close();
                curCSV.close();
            } catch (Exception sqlEx) {
                Log.e("ResultActivity", sqlEx.getMessage(), sqlEx);
            }
        }
    });
}
}

Answer 1

您的變量名似乎犯了一個錯誤：實例化對象db

 DatabaseHelper db = new DatabaseHelper(getApplicationContext());

但是您在下面使用dbhelper

SQLiteDatabase myDb = dbhelper.getReadableDatabase();

尚未初始化，並拋出NullPointer。

將上面的行更改為SQLiteDatabase myDb = db.getReadableDatabase();