Joda-Time: what's the difference between Period, Interval and Duration?

Question

In Joda-Time 2, what is the difference between the three kinds of time spans:

• Period
• Interval

• Duration

  1. Why do we need th

Answer 1

To add to mikej's answer:

A Joda-Time duration is a "physical" time interval; eg:

12000 milliseconds <-- this is a duration

A Joda-Time interval is actually a pair of instants (start instant - end instant). An instant is, again, a "physical" concept, a point in the timeline. Eg (just a possible notation):

(2010/3/3 19:00:00.000 UTC ; 2010/3/3 20:00:00.000 UTC) <-- this is an interval

An interval, then, can be converted to a duration, but not the reverse.

Consider these two intervals:

I1=(2010/3/3 19:00:00.000 UTC ; 2010/3/3 20:00:00.000 UTC)

I2=(2010/3/3 21:00:00.000 UTC ; 2010/3/3 22:00:00.000 UTC)

As intervals, I1 and I2 are different, because the end-points are different; but if I convert them to durations, I get the same thing: 3600000 milliseconds.

(Math analogy: the intervals [10,12] and [95,97] are different intervals, but they have the same length: "interval length" maps to duration).

Finally, a period is a lapse of "civil time", expressed as a number of months, days, hours, etc. It does not -by itself- represent a "physical" interval, hence it can't be directly converted to a duration (months have variable lengths...).

This answers question 3: you can only divide by two a physical time (a duration).