在SQLiteDatabase安卓系统中增加表数时清除模拟器数据的必要性

Question

我有一个小小的疑问，我没有得到满意的答案。以下是场景：

当我增加已经存在的数据库中的表数时，我在OpenHelper类的onCreate方法中得到了一个NPE，但是当我在清除模拟器数据之后运行相同的应用程序时，一切似乎都奇迹般地运行得很好……

我不知道为什么会发生这种情况，任何解释都会有帮助...

public static final String DATABASE_NAME = "MONEY_DATABASE";
public static final String TABLE_NAME = "MONEY_TABLE";
public static final String TABLE_FIELDS = "FIELD_TABLE";

public static final int DATABASE_VERSION = 4;

public static final String KEY_ID = "_id";
public static final String KEY_AMOUNT = "amount";
public static final String KEY_DATE = "date_int";

public static final String KEY_TYPE_ID = "type_id";
public static final String KEY_TYPE = "type";
public static final String KEY_TITLE = "field_title";

Context localContext;

public HelperMethods (Context c)
{
    Log.d("dev","HelperMethods object created");
    this.localContext = c;
    localopenhelper = new OpenHelper(localContext);

    if(localContext == null)
    {
        Log.d("dev","localContext is null");
    }

}


public SQLiteDatabase database ;

class OpenHelper extends SQLiteOpenHelper
{
    // CREATE TABLE MONEY_TABLE ( _id INTEGER PRIMARY KEY AUTOINCREMENT, amount DOUBLE);

    private String query = "CREATE TABLE " + TABLE_NAME + " ( " + KEY_ID +  " INTEGER PRIMARY KEY AUTOINCREMENT, "
            + KEY_AMOUNT + " DOUBLE, " + KEY_DATE + " DOUBLE);";

    private String query1 = "CREATE TABLE " + TABLE_FIELDS + " ( " + KEY_TYPE_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
           + KEY_TYPE + " TEXT NOT NULL, " + KEY_TITLE + " TEXT NOT NULL);";

    Context helperContext;

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

        this.helperContext = context;

    }

    @Override
    public void onCreate(SQLiteDatabase db)
    {
        // NPE occurs exactly at this line..
        db.execSQL(query);

        db.execSQL(query1);
    }

还有关于LogCat的东西：

Process: com.giridharkarnik.moneymanager, PID: 1149
java.lang.NullPointerException
        at com.giridharkarnik.moneymanager.HelperMethods$OpenHelper.onCreate(HelperMethods.java:77)
        at com.giridharkarnik.moneymanager.HelperMethods$OpenHelper.onUpgrade(HelperMethods.java:88)

Answer 1

从堆栈跟踪可以看出，您的onUpgrade()包含如下内容：

onCreate(null);

您还需要将获得的数据库作为参数传递给onCreate()的onUpgrade()。