# Java Notes

# Various time classes

Solving all the date and time problems is a tough job (different calendar systems, time zones, date formats, date arithmetic, leap seconds, ...). You will find several classes useful for handling times and dates.

- `java.util.Date` - A common representation of dates.
- `java.util.Calendar` - From the Java documentation: "Calendar is an abstract base class for converting between a Date object and a set of integer fields such as YEAR, MONTH, DAY, HOUR, and so on." This is a very useful class, and you will often use this in preference to Date in my (limited) experience.
- `java.text.SimpleDateFormat` - Useful for both parsing and formatting dates.

There are a lot of details in some areas and you will want to consult the JDK documentation for the specifics, but here are a few simple solutions to common problems.

## Time in milliseconds since January 1, 1970

If you aren't familiar with Unix, this may sound a little strange, but it is the common Unix way of measuring time and it has be adopted by Java. If you're just measuring elapsed time, you can call `System.currentTimeMillis()`, which avoids the overhead of creating a Calendar object. This returns a `long` integer value. For example,

    long startTime = System.currentTimeMillis();

This is not a useful way to get a time or date that is readable by humans, but is fine for figuring out how many seconds or milliseconds something took.

## Getting a printable date and time

### Simple, but ugly

To get a string which has the current date and time in a fairly readable format, use the default conversion to String.

    String rightNow = "" + new Date();

This produces something like "Sun May 02 21:49:02 GMT-05:00 1999". It's not pretty, but it is a quick way to display the time.

### Formatting the date

To format a date, use the `java.text.SimpleDateFormat` class. See the Java API documentation for a good description of this class.

## Using the `java.util.Calendar` class

The Calendar class can be used to get the current date and time, represent dates and times for the past or future, do date arithmetic, determine the hour, day of the week, year, etc. This, as well as `java.util.Date` are the common classes to use.

       Calendar today = new GregorianCalendar(); // Current date and time.

Because Calendar is an abstract class, you must create a specific kind of calendar to assign to it. The only supported subclass is `java.util.GregorianCalendar`, which is the calendar that is commonly used

## Using a Timer

You can use a javax.swing.Timer object to call a method of yours at regular intervals. This is useful for many things, including animation. The Timer class is easy to use - it calls its action listeners at specified intervals.