Parse String to Date and then compare Dates

Question

I am working on a Adaptive To-Do application, In this user will have a button to reschedule the pending tasks(Tasks with past date), when the user clicks reschedule, it'll change the pending tasks date to current free date(Dates which are not assigned to any task. What i cant understand is how to retrieve date from database and compare it with the available dates.But formater shows error, and if comparision also.

public void UpdateData()  {
    Calendar cal = Calendar.getInstance();
    SQLiteDatabase db = mHelper.getReadableDatabase();
    Cursor cursor = db.query(TaskContract.TaskEntry.TABLE_NAME,
            new String[]{TaskContract.TaskEntry._ID,
                    TaskContract.TaskEntry.COLUMN_DATE,
            },
            null, null, null, null, null);
    //Calendar reminderCalendar = Calendar.getInstance();
    //reminderCalendar.set(year, month, day);
    SimpleDateFormat formatter  = new SimpleDateFormat("dd/MM/yyyy");
    String str1 = TaskContract.TaskEntry.COLUMN_DATE;
    Date date1 = formatter.parse(str1);//Show Error UnHandled Exception java.text.parse exception.
    if (date1.before(cal))// Shows error and says to flip to (cal.before(date1)) {

        ContentValues cv = new ContentValues();
        final Calendar c = Calendar.getInstance();
        int yy = c.get(Calendar.YEAR);
        int mm = c.get(Calendar.MONTH);
        int dd = c.get(Calendar.DAY_OF_MONTH);

    }

}

Answer 1

String str1 = TaskContract.TaskEntry.COLUMN_DATE;

Assuming your column is called MYDATE, your code is saying that str1 = "MYDATE"; Can you tell me what date MYDATE is? Nor can the formatter, hence the exception.

I believe what you want is:-

        String str1 = "";
        if (cursor.moveTofirst) {
            str1 = cursor.getString(cursor.getColumnIndex(TaskContract.TaskEntry.COLUMN_DATE));
        } else {
            // handle no data being extracted for some reason
        }
        cursor.close(); // assuming you've done with the Cursor
        Date date1 = formatter.parse(str1); 
        ..........

To further explain when you say :-

`Cursor cursor = db.query(......`

You are saying get me a cursor from the table(1st parameter), with the columns (2nd parameter, null for all columns), 3rd parameter where clause and so on.

If it works you will get a Cursor named cursor which contain your data as a number of rows of columns (like a spreadhsheet).

To get to the data you need to do the equivalent of get the data from cell A1 (A being the first column of the first row).

However you move to a Row and then get the nth column using one the get???? methods. Above move's to the first row (we'd only expect the 1 row by the looks of your code (more later)).

At a row you can get data from columns by using the index/offset to that column. The first column being 0, the next 1 etc.

However, having to work this out and perhaps change it if you change the query can introduce difficulties, so it's probably much easier to find the index/offest by using the column name. Hence, getColumnIndex(column_name_as_a_string)

Back to moving, to explain why the moveToFirst is in an if.

A cursor can contain 0 or more rows, a cursor, if the query works will not be null, rather an empty cursor is returned (this is a relatively frequent occurrence, and also sometimes a very useful situation).

So rather than issuing an exception because a Cursor can't move to a position, it returns false (true if it can and does move).

So if(cursor.moveToFirst) {....} is saying only do if there is data in the cursor.

There are other move methods, moveToLast(), moveToNext(), moveToPrevious() and moveToPosition(int).

Perhaps the most common is moveToNext e.g.

while(cursor.moveToNext()) {
    // use the respective column data here
}

traversing all rows of the cursor