Question

I am making inner static class as a singleton to avoid A SQLiteConnection object for database 'db' was leaked! Please fix your application to end transactions in progress properly and to close the database when it is no longer needed.

I can close the mDb.close after db transactions using mDb instance(in case of just static class) ,

I want to know is it right way to make inner static class as a singleton in a constructor of outer class ??

  private ImageDb mDb;

        public class SavedImages extends BaseAdapter implements ListAdapter {
             public SavedImages(Activity context) {
                    ImageDb.moveFromCacheDirectoryIfNecessary(context);
                    mDb = ImageDb.getInstance(context);

                }


    static class ImageDb extends SQLiteOpenHelper {
        final static int DB_VERSION = 1;
        final static String DB_NAME = "wallpaper_images.db";
        final static String TABLE_NAME = "wallpaper_images";
        final static String COLUMN_ID = "id";
        final static String COLUMN_IMAGE_THUMBNAIL_FILENAME = "image_thumbnail";
        final static String COLUMN_IMAGE_FILENAME = "image";

        //Context mContext;

        private static ImageDb mImageDb;

    public static ImageDb getInstance(Context context) {
        if (mImageDb == null) {
            mImageDb = new ImageDb(context.getApplicationContext());
        }
        return mImageDb;
    }


    private ImageDb(Context context) {
        super(context, context.getDatabasePath(DB_NAME).getPath(), null, DB_VERSION);
         }

        public static void moveFromCacheDirectoryIfNecessary(Context context) {
           //static method
        }
        @Override
        public void onCreate(SQLiteDatabase database) {
   //oncreate db
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
          //onupgrade db
        }
    }
 }

Answer 1

实现此目的的正确方法是使文件具有名为A

的类

public class A {
  private static class SingletoneHolder {
    //here you declare the instance of what you want
  }

  public static YourInstance getInstance(){
    return YourInstance;
  }
}

这样，只要您想在应用内部访问，就可以访问此类的实例。

Answer 2

class TestOuter2{  

    static int data=30;  

    static class Inner{  
        static void msg() {
            System.out.println("data is "+data);
        }  
    } 

    public static void main(String args[]){  
        TestOuter2.Inner.msg();//no need to create the instance of static nested class  
    }  

}

Can i make inner static class as a singleton

2 个答案: