How does Cursor work in android [closed]

问题:

I'm having a hard time exactly visualizing 'Cursor' functionality in my program. I kind of get the jist of it, but can anyone explain it's functionality in detail?

By Cursor, I mean the Cursor interface. I can't simply understand the role it plays with anything.

http://developer.android.com/reference/android/database/Cursor.html

回答1:

A Cursor object is returned from a query on a SQLite database. It will return all rows that the query returns.

Say you have a table called names in your database database configured as such:

_id     _name
1       Space Ghost
2       Zorak
3       Moltar
4       Brak

If you want to get all data from this table and use it, you would do something like this:

public HashMap<Integer, String> getNames(){

 HashMap<Integer, String> data = new HashMap<Integer, String>();

 try{
  SQLiteOpenHelper helper = new MyOpenDbHelper(context);
  SQLiteDatabase db = helper.getReadableDatabase();
  String selectQuery = "SELECT  * FROM names";
  Cursor cursor = db.rawQuery(selectQuery, null);
  if (cursor != null && cursor.moveToFirst()){ //make sure you got results, and move to first row
    do{
        int mID = cursor.getInt(0); //column 0 for the current row
        String mName = cursor.getString(1); //column 1 for the current row
        data.put(mID, mName);

      } while (cursor.moveToNext()); //move to next row in the query result

  }

 } catch (Exception ex) {
    Log.e("MyApp", ex.getMessage());
 } finally
 {
    if (cursor != null) {
       cursor.close();

    }
    if (db != null) {
        db.close();
    }

 }

return data;
}

Usually you will create your own class to extend SQLiteOpenHelper, as such:

public class MyOpenDbHelper extends SQLiteOpenHelper {
     //........
}

回答2:

From Wikipedia

In computer science, a database cursor is a control structure that enables traversal over the records in a database. Cursors facilitate subsequent processing in conjunction with the traversal, such as retrieval, addition and removal of database records. The database cursor characteristic of traversal makes cursors akin to the programming language concept of iterator.

From Here

A cursor is a tool that allows you to iterate the records in a set. It has concepts of order and current record.

From The documentation you pointed yourself

provides random read-write access to the result set returned by a database query.

So don' t think Cursor as a functionality, but as a mean for reaching records in a more efficient way from any database.

回答3:

Are you reffering to this Cursor usage?

public String databaseToString(){
    String dbString = "";
    SQLiteDatabase db = getWritableDatabase();
    String query = "SELECT * FROM " + TABLE_PRODUCTS + " WHERE 1";

    //Cursor points to a location in your results
    Cursor c = db.rawQuery(query, null);
    //Move to the first row in your results
    c.moveToFirst();