Question

我知道这是一个预先问过的问题。但我仍然无法找到解决方案我正在使用SQLiteOpenHelper打开一个数据库，并在SQLiteDatabase对象中应用插入和查询方法但我在查询方法中得到NullPointerException -
这是我的openhelper课程：

public class MovieDbHelper extends SQLiteOpenHelper {
    private static final int DATABASE_VERSION = 2;

    static final String DATABASE_NAME = "movie.db";
    public MovieDbHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }


    @Override
    public void onCreate(SQLiteDatabase sqLiteDatabase) {
           String create_mov_table = "CREATE TABLE" + "MOVIE" + "(" +
                                    "mov_id"  +  " INTEGER PRIMARY KEY AUTOINCREMENT," +
                                   "mov_pos"  +  " BLOB);";
sqLiteDatabase.execSQL(create_mov_table);

    }

    @Override
    public void onUpgrade(SQLiteDatabase sqLiteDatabase, int i, int i1) {
        // should be your top priority before modifying this method.
        sqLiteDatabase.execSQL("DROP TABLE IF EXISTS " + "MOVIE");

        onCreate(sqLiteDatabase);
    }
}

这是我的实用工具类，我在其中应用方法 -

public class utility extends ActionBarActivity {
   static Context c ;
    // convert from bitmap to
    // byte array
   static SQLiteDatabase db;
    public void onCreate(){

        utility.c = getApplicationContext();
        MovieDbHelper mhelper = new MovieDbHelper(c);

         db = mhelper.getWritableDatabase();

    }

    public static Context getAppContext() {
        return utility.c;
    }


    public static byte[] getBytes(Bitmap bitmap) {
        ByteArrayOutputStream stream = new ByteArrayOutputStream();
        bitmap.compress(Bitmap.CompressFormat.JPEG, 0, stream);
        return stream.toByteArray();
    }

    // convert from byte array to bitmap
    public static Bitmap getImage(byte[] image) {
        return BitmapFactory.decodeByteArray(image, 0, image.length);
    }
    public static void addEntry( byte[] image) throws SQLiteException {

        ContentValues cv = new  ContentValues();

        cv.put("mov_pos",   image);
        db.insert( "MOVIE", null, cv );
    }
    public static Cursor getEntry(String[] columns){
        return db.query("MOVIE",columns,null,null,null,null,null);  **--  nullpointer** 
    }

}

我执行：

Cursor cup;
 String[] cm = {"mov_pos"};
            cup = utility.getEntry(cm);  **-- null pointer**

Answer 1

由于您从扩展Activity的类中调用静态方法，因此该类的onCreate方法显然未被调用，这就是您获得NPE的原因。

首先将实用程序类中的所有方法移动到MovieDbHelper类，然后使用构造函数中提供的context。

public class MovieDbHelper extends SQLiteOpenHelper {
    private Context context;
    private static final int DATABASE_VERSION = 2;
    static final String DATABASE_NAME = "movie.db";

    public MovieDbHelper(Context context) {
         super(context, DATABASE_NAME, null, DATABASE_VERSION);
         this.context = context;
    }
}

例如，您的getEntry（将其放在MovieDbHelper中）可以重写为：

public Cursor getEntry(String[] columns){
    SQLiteDatabase db = this.getReadableDatabase();
    return db.query("MOVIE",columns,null,null,null,null,null);  **--  nullpointer** 
}

编辑：

在您的活动中使用：

MovieDbHelper helper = new MovieDbHelper(YourActivityName.this);
String[] cm = {"mov_pos"};
Cursor c = helper.getEntry(cm);

片段中的用法：

MovieDbHelper helper = new MovieDbHelper(getActivity().getApplicationContext());
String[] cm = {"mov_pos"};
Cursor c = helper.getEntry(cm);

编辑2：

public addEntry( byte[] image) throws SQLiteException {
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues cv = new  ContentValues();

    cv.put("mov_pos",   image);

    try {
        db.insertOrThrow("MOVIE", null, cv );
    } catch (SQLiteConstraintException e) {
        e.printStackTrace();
        return false;
    }
    return true;
}

在Sqlitedatabase.query方法中获取空指针异常

1 个答案: