Java-在方法中将Cursor变量设为静态

Question

我已经用Java创建了一个mySQL数据库，而我想做的就是让该类在'getData（）'方法中只有'Cursor c'变量的一个实例。 这样一来，只要调用getData()方法，游标就可以从中断处继续读取数据库，而不是从头开始。

我尝试将“ Cursor c”从方法中删除，使其固定为静态，但随后导致其他问题。

你能告诉我如何正确地做吗？

    public class SQLClass {

    public static final String KEY_ROWID = "_id";
    public static final String KEY_NAME = "IncomeName";
    public static final String KEY_AMOUNT = "IncomeAmount";

    private static String DATABASE_NAME = "MoneyTracker";
    private static String DATABASE_TABLE = "IncomeTable";
    private static int DATABASE_VERSION = 1;


    private DbHelper ourHelper;
    private final Context ourContext;
    private SQLiteDatabase ourDatabase;

    private static class DbHelper extends SQLiteOpenHelper{

        public DbHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
            // TODO Auto-generated constructor stub
        }//end DbHelper constructor

        @Override
        public void onCreate(SQLiteDatabase db) {
            // TODO Auto-generated method stub
            db.execSQL("CREATE TABLE " + DATABASE_TABLE + "(" + KEY_ROWID + "         INTEGER PRIMARY KEY AUTOINCREMENT, "
                    + KEY_NAME + " TEXT NOT NULL, " + KEY_AMOUNT + " TEXT NOT NULL);");
        }//end onCreate method 

        @Override
        public void onUpgrade(SQLiteDatabase db, int arg1, int arg2) {
            // TODO Auto-generated method stub
            db.execSQL("DROP IF EXISTS " + DATABASE_NAME);
            onCreate(db);
        }//end onUpgrade method

    }//end class DbHelper

    public SQLClass (Context c){
        ourContext = c;
    }//end constructor SQLClass

    public SQLClass open()throws SQLException{
        ourHelper = new DbHelper(ourContext);
        ourDatabase = ourHelper.getWritableDatabase();
        return this;
    }//end method open

    public void close(){
        ourHelper.close();
    }//end method close

    public long createEntry(String name, String amount) {
        // TODO Auto-generated method stub
        ContentValues cv = new ContentValues();
        cv.put(KEY_NAME, name);
        cv.put(KEY_AMOUNT, amount);
        return ourDatabase.insert(DATABASE_TABLE, null, cv);
    }


    public String[] getData() {
        // TODO Auto-generated method stub
        String[] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_AMOUNT};
        Cursor c = ourDatabase.query(DATABASE_TABLE, columns, null, null, null, null, null);
        String[] result = new String[]{"","",""};

        int iRow = c.getColumnIndex(KEY_ROWID);
        int iName = c.getColumnIndex(KEY_NAME);
        int iAmount = c.getColumnIndex(KEY_AMOUNT);


            c.moveToNext();
            result[0] = c.getString(iRow);
            result[1] = c.getString(iName);
            result[2] = c.getString(iAmount);

        return result;
    }//end method getData
}//end class SQLClass

Answer 1

这是一个经典的Java示例，如何在非静态类方法中访问静态字段：

public class MyClass {

        private static int staticCursor;

        {  // Initialize 
            staticCursor = 1;
        }

        public int getData(){
            int retval;
            retval = MyClass.staticCursor;
            MyClass.staticCursor++;
            return retval;
        }

        public static void main(String[] args) {

            MyClass myClass1 = new MyClass();
            MyClass myClass2 = new MyClass();
            MyClass myClass3 = new MyClass();

            System.out.println( myClass1.getData() );
            System.out.println( myClass2.getData() );
            System.out.println( myClass3.getData() );
            System.out.println( myClass1.getData() );
      }
    }

run results:
====================
1
2
3
4

但是，如果计划在多线程环境中运行的许多类实例之间共享游标变量，则需要仔细考虑的同步和初始化工作，线程之间共享游标会导致难以诊断的错误。

Answer 2

谢谢您帮助我了解了如何以非静态方法访问静态字段，但这不是我想要的。 也许我没有问正确的问题。 无论如何，我设法通过使getData（）接受参数int position作为游标的当前位置并将c.moveToNext（）更改为c.moveToPosition（position）来解决问题。

        public String[] getData(int position) {
    // TODO Auto-generated method stub
    String[] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_AMOUNT};
    Cursor c = ourDatabase.query(DATABASE_TABLE, columns, null, null, null, null, null);
    String[] result = new String[]{"","",""};

    int iRow = c.getColumnIndex(KEY_ROWID);
    int iName = c.getColumnIndex(KEY_NAME);
    int iAmount = c.getColumnIndex(KEY_AMOUNT);


    c.moveToPosition(position);
    if (!c.isAfterLast()){
    result[0] = c.getString(iRow);
    result[1] = c.getString(iName);
    result[2] = c.getString(iAmount);

    }else if(c.isAfterLast()){
    result[0] = "false";

    }

    return result;
}//end method getData