0

我用java创建了一个mySQL数据库，我想做的是让类只有一个'getData（）'方法中的'Cursor c'变量实例。这样一来，无论何时getData()调用该方法，游标都可以从上次停止的地方继续读取数据库，而不是从头开始。

我尝试从方法中取出“Cursor c”并将其设为静态，但随后会导致其他问题。

你能告诉我如何正确地做到这一点吗？

    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
4

2 回答 2

0

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，如果您计划在多线程环境中运行的多个类实例之间共享游标变量，则需要仔细考虑同步和初始化的东西，在线程之间共享游标可能会导致难以诊断错误。

于 2013-07-27T08:34:36.827 回答
0

谢谢你帮助我了解如何在非静态方法中访问静态字段，但这不是我想要的。也许我没有问正确的问题。无论如何，我设法通过让 getData() 接受一个参数 int position 来解决问题，用作光标的当前位置并将 c.moveToNext() 更改为 c.moveToPosition(position)

        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
于 2013-07-28T14:02:49.503 回答