无法找到数据库文件

Question

我正在尝试使用dataBAseHelper类来创建数据库文件，但我无法在数据文件中找到它（它仍然是空的）。在调试时，似乎没有调用onCreate方法。我尝试了不同的databaseHelper版本，但它没有用。 你可以在下面看到我的代码（MAinActivity和DataBaseHelper类）：

The DatabaseHelper class

public class DatabaseHelper extends SQLiteOpenHelper {
    public static final String DATABASE_NAME = "tasks.db";
    public static final String TABLE_NAME = "task_table";
    public static final String ID_COL = "ID";
    public static final String NAME_COL = "NAME";
    public static final String CATEGORY_COL = "CATEGORY";
    public static final String DURATION_COL = "DURATION";
    public static final String DATETIME_COL = "DATETIME";
    public static final String DESCRIPTION_COL = "DATETIME";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_TABLE_TASKS = "CREATE TABLE " + TABLE_NAME  + "("
                + ID_COL + " INTEGER PRIMARY KEY AUTOINCREMENT ,"
                + NAME_COL + " TEXT, "
                + DATETIME_COL + " TEXT,"
                + DURATION_COL + " INTEGER,"
                + CATEGORY_COL + " TEXT,"
                + DESCRIPTION_COL + " TEXT " + ")";
        db.execSQL(CREATE_TABLE_TASKS);
    }

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

    public boolean insertData(Task task) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put(NAME_COL,task.getName());
        contentValues.put(CATEGORY_COL,task.getCategory().toString());
        contentValues.put(DATETIME_COL,task.getDateTime());
        contentValues.put(DURATION_COL,task.getDuration());
        contentValues.put(DESCRIPTION_COL,task.getDescription());

        long result = db.insert(TABLE_NAME,null ,contentValues);
        if(result == -1)
            return false;
        else
            return true;
    }

    public Cursor getAllData() {
        SQLiteDatabase db = this.getWritableDatabase();
        Cursor res = db.rawQuery("select * from "+TABLE_NAME,null);
        return res;
    }

    public boolean updateData(String id,Task task) {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put(ID_COL,id);
        contentValues.put(CATEGORY_COL,task.getCategory().toString());
        contentValues.put(DATETIME_COL,task.getDateTime());
        contentValues.put(DURATION_COL,task.getDuration());
        contentValues.put(DESCRIPTION_COL,task.getDescription());
        db.update(TABLE_NAME, contentValues, "ID = ?",new String[] { id });
        return true;
    }

    public Integer deleteData (String id) {
        SQLiteDatabase db = this.getWritableDatabase();
        return db.delete(TABLE_NAME, "ID = ?",new String[] {id});
    }
}

The MainActivity class

public class MainActivity extends AppCompatActivity {
    DatabaseHelper data;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
         data = new DatabaseHelper(this);
        setContentView(R.layout.activity_main);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu){

    getMenuInflater().inflate(R.menu.item,menu);
    Intent intent = new Intent(MainActivity.this, addActivity.class);
   // startActivity(intent);
    return super.onCreateOptionsMenu(menu);
}


    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.add:

                Intent  intent = new Intent(MainActivity.this,addActivity.class);
                startActivity(intent);
        }
        return super.onOptionsItemSelected(item);
    }
}

Answer 1

你实际上必须对数据库做一些调用onCreate方法的事情;而不是仅仅创建数据库帮助程序的实例（更具体地说，使用getWriteableDatabase或getReadableDatabase执行打开数据库的操作）。

所以尝试添加一行如data.insert(new task(paremateres_to_construct_the task);

或者，您可以更改databasehelper构造函数以调用其中一个方法，从而导致调用onCreate方法。例如： -

public DatabaseHelper(Context context) {
    super(context, DATABASE_NAME, null, 1);
    this.getWriteableDatabase();
}

后者将创建一个空表，前者将创建一个表添加一行。两者都不是问题。