I believe no matter what kind of application you are developing. You could not avoid time manipulation. In Java, we use native java.util.Calendar and java.util.Date to handle date and time problem. Due to their bad design and interface, we have a potential third party library  called Joda Time which really make our life easier. It is open source and maturity which has been under active development since 2002.

For a date creation example, when we use native Calendar class:

Calendar calendar = Calendar.getInstance();
calendar.set(2000, Calendar.JANUARY, 1, 0, 0, 0);

Using Joda, the code looks like this:

DateTime dateTime = new DateTime(2000, 1, 1, 0, 0, 0, 0);

We could use number to present month and 1 means January.

So what about modifying date object, let’s say we add 30 days to a certain date, the JDK way:

Calendar calendar = Calendar.getInstance();
calendar.set(2010, Calendar.JANUARY, 1, 0, 0, 0);SimpleDateFormat sdf =  new SimpleDateFormat("E MM/dd/yyyy HH:mm:ss.SSS");calendar.add(Calendar.DAY_OF_MONTH, 30);System.out.println(sdf.format(calendar.getTime()));

Using Joda, two lines of code to do the same thing:

DateTime dateTime = new DateTime(2000, 1, 1, 0, 0, 0, 0); System.out.println(dateTime.plusDays(30).toString("E MM/dd/yyyy HH:mm:ss.SSS");

We save some lines of code to create Calendar and SimpleDateFormat instance. And one importance concept of Joda is that, the instance is immutable. So anytime you call a method for a change. It will return a new DateTime instance, and a handy “toString” method for easy formatting output.

Of course, Joda is more than a fancy interface library. It includes more features:

• ReadablePartial: Sometime you just care the year/month/day like birthday, or the time of day. You could use LocalDate and LocalTime to represent those information without the complex of time zone
• Spans of time concept: Duration,Period and Interval classes represent an absolute mathematical span, expressed in milliseconds (such as 60 seconds in a minute and 24 hours in a day) to be used to date manipulation.
• Timezone support.
• multiple times (that is, repeat) in a meaningful way, then it is a partial.
• Chronology: different kind of calendar support: ISO (the default), Coptic, Julian, Islamic. The library currently provides 8 calendar systems.
• Better Performance Characteristics.

To know more about this project, the official homepage is here:

http://joda-time.sourceforge.net/

Practical usage of Joda is here:

http://www.ibm.com/developerworks/java/library/j-jodatime.html

Noda Time is a port of Joda Time to .NET, here is an article from the code project to help you get started.

If you’re a Java programmer, this short article will show you the basic concepts of functional programming and the reason why it may be getting more and more important in the near future. All sample code is in Java so it should be very easy to understand.

What is functional programming?

Functional programming is just like our programming in Java or C#, but without assignments. You may wonder is this possible at all? For example, to calculate the total price of a list of product prices, one might write in functional programming style (the code is still in Java):

abstract class List {
}

class EmptyList extends List {
}

class NonEmptyList extends List {
  int head;
  List tail;
}

class Foo {
  int getTotalPrice(List ps) {
    if (ps instanceof EmptyList) {
      return 0;
    } else {
      final NonEmptyList nl = (NonEmptyList) ps; //note1: initialization, NOT assignment
      return nl.head + getTotalPrice(nl.tail);
    }
  }
}

Note that the highlighted code below seems to be an assignment, but it is not: It is an initialization and the variable is declared as final, so no modification can be made. This is perfectly allowed in functional programming.

So you can see that assignment is not really required. But why the lack of assignment is a good thing?

To see the benefit, let’s assume that if a price is >= 100 then you’ll give a 20% off discount. So, you may modify the code as:

class Foo {
int getTotalPrice(List ps) {
  if (ps instanceof EmptyList) {
    return 0;
  } else {
    final NonEmptyList nl = (NonEmptyList) ps;
    final int headDiscountedPrice = nl.head >= 100 ? (int) (nl.head * 0.8) : nl.head;
    final int tailPrice = getTotalPrice(nl.tail);
    return headDiscountedPrice + tailPrice;
  }
}
}

Note the three initializations. Because all the variables and fields can't be modified, their order is unimportant and, in a real functional programming language, the order can be changed at wish without changing the return value. For example, you could change it like:


class Foo {
int getTotalPrice(List ps) {
  if (ps instanceof EmptyList) {
    return 0;
  } else {
    final int tailPrice = getTotalPrice(nl.tail); //nl is not initialized yet, how it works?
    final int headDiscountedPrice = nl.head >= 100 ? (int) (nl.head * 0.8) : nl.head;
    final NonEmptyList nl = (NonEmptyList) ps;
    return headDiscountedPrice + tailPrice;
  }
}
}

Note that when initializing the tailPrice variable, the nl variable hasn’t been initialized yet. Will this cause a problem? No. In a real functional programming language, each of three variables will be initialized at the same time with a lazy expression. When the value is really needed, the lazy expression will be evaluated. So, if the value of tailPrice is needed but nl hasn’t been evaluated yet, it will be evaluated and the calculation will proceed. No matter what execution order is, the final total price will be the same.

 

Now, let’s get to the core issue of why this is important. As the order of these expressions are unimportant, they can be evaluated concurrently. As nowadays we’re getting more CPU cores instead of speedier single CPU, this programming model may become the mainstream in the future as the evaluations of different expressions can be done in different cores.

 

In summary, there are some basic concepts in functional programing:

●    Lazy evaluation

●    Functions return values

○     Given a set of values in the parameter list, the function can only have one possible result.

●    Functions have no side-effects

○     Cannot modify variables passed to them

○     Cannot modify any global variable

○     Variables only assigned once

 

 

If you’d like to know more how the procedure programing pattern could be applied in it ( such as: sequential execution or looping ), you could check this full paragraph of this article : Learn the basic concepts and significance of functional programming in 10 minutes

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