Java: Various time classes

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Java: 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

long startTime = System.currentTimeMillis();

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

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Java: Methods - Introduction

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Methods

The word method is commonly used in Object-Oriented Programming and is used in Java. It means the same thing as function, procedure, or subroutine in other programming languages. Many programmers use these other terms, especially function, but these notes will use method

Why use methods?

For reusable code

If you need to do the same thing, or almost the same thing, many times, write a method to do it, then call the method each time you have to do that task.

To parameterize code

In addition to making reusable code that is the same in all cases, you will often want to use parameters that change the way the method works.

For top-down programming

A very useful style of programming is called top-down programming. You solve a big problem (the "top") by breaking it down into little problems. To do this in a program, you write a method for solving your big problem by calling on other methods to solve the smaller parts of the problem. And these methods for solving the simpler problems similarly call on other methods until you get down to simple methods which solve simple problems

To create conceptual units

Create methods to do something that is one action in your mental view of the problem. This will make it much easier for you to work with your programs.

To simplify

Because local variables and statements of a method can not been seen from outside the method, they (and their complexity) are hidden from other parts of the program, which prevents accidental errors or confusion.

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