Question

I’m writing an application where users enter some details into an SQLite Database. I need to write code whereby the last 31 days of stored data in the database is outputted.

The code to save the date string is:
currentDateString = DateFormat.getDateInstance().format(new Date());

This gives the date in the format (for todays date) as 30 Mar 2012.

Is there anyway to change this data format into DD/MM/YYYY? (30/03/2012)

This string is saved into the SQL Database as follows:

ContentValues dbcv = new ContentValues();
dbcv.put(KEY_DATE, date);

(currentDateTime String = Date in previous class)

I wish to output every row of the database that comes within the last 31 days.

How can I do this?

Thanks!

Answer 1

First of all, you are doing two questions instead on just one, I will try to give you an answer to both.

1. To change the date format in java take a look at the documentation for the SimpleDateFormat, you can pass a pattern to the constructor of that class to make it format your dates the way you really want. In your specific case the pattern you are looking for is dd/MM/yyyy
2. If you are storing dates in your database as strings you lose some of the benefits that storing dates as they are have. For example, your second question will be easily performed if you were storing dates instead of strings in your tables since the only thing you need to achieve what you want is to do a select operation to your table using the date functions provided by SQLLite. Take a look a this webpage as it will give you an idea of what you can do with those functions: SQLLite Date Functions

Note: Of course, if you still want to store them as strings the only solution you will have is read all the records, parse all the strings back to dates and use the java.util.Calendar class to check which record is older than 31 days and which one is not. I’d rather do the SQL query instead of that as it has too much better performance (specially on a mobile device).

EDIT:

SQLLite and Android are not specifically my strengths, I did a small research and I found several posts in which people were actually storing them as strings as you initially started to do. If SQLLite on Android doesn’t support storing dates as they are (as other databases do) then maybe the best option in your scenario is using a long value as the date. Look at the Date definition class, it has a method getTime that returns the date value as long value, you can use that value later to create a new instance of a date (using its constructor).

To format that date (the Date object, of course) in something readable by the user, DateFormat (or any of its implementations) is still the answer.

To do the query you want to do having dates stored as longs I suggest you to take a look to the DateUtils class from the Apache Commons Lang library. In that class you will find several functions that work on dates, so, before creating the query you need to fetch the rows in last 31 dates, you could use something like:

Date now = new Date();
Date before = DateUtils.addDays(now, -31);
long beforeAsLong = before.getTime();
String query = "select * from TABLE_NAME where DATE_FIELD >= " + beforeAsLong;

Hope this helps.