Java的Calendar和Date类
Java 语言的Calendar(日历),Date(日期)，和DateFormat(日期格式)组成了Java标准的一个基本但是非常重要的部分。日期是商业逻辑计算一个关键的部分。所有的开发者都应该能够计算未来的日期，定制日期的显示格式，并将文本数据解析成日期对象。
创建一个日期对象
让我们看一个使用系统的当前日期和时间创建一个日期对象并返回一个长整数。这个时间通常被称为Java 虚拟机(JVM)主机环境的系统时间。
import java.util.Date; public class DateExample1 { public static void main(String[] args) { // Get the system date/time Date date = new Date(); System.out.println(date.getTime()); } } import java.text.SimpleDateFormat; import java.util.Date; public class DateExample2 { public static void main(String[] args) { SimpleDateFormat bartDateFormat = new SimpleDateFormat("EEEE-MMMM-dd-yyyy"); Date date = new Date(); System.out.println(bartDateFormat.format(date)); } } import java.text.SimpleDateFormat; import java.util.Date; public class DateExample3 { public static void main(String[] args) { // Create a date formatter that can parse dates of // the form MM-dd-yyyy. SimpleDateFormat bartDateFormat = new SimpleDateFormat("MM-dd-yyyy"); // Create a string containing a text date to be parsed. String dateStringToParse = "9-29-2001"; try { // Parse the text version of the date. // We have to perform the parse method in a // try-catch construct in case dateStringToParse // does not contain a date in the format we are expecting. Date date = bartDateFormat.parse(dateStringToParse); // Now send the parsed date as a long value // to the system output. System.out.println(date.getTime()); } catch (Exception ex) { System.out.println(ex.getMessage()); } } } import java.text.DateFormat; import java.util.Date; public class DateExample4 { public static void main(String[] args) { Date date = new Date(); DateFormat shortDateFormat = DateFormat.getDateTimeInstance( DateFormat.SHORT, DateFormat.SHORT); DateFormat mediumDateFormat = DateFormat.getDateTimeInstance( DateFormat.MEDIUM, DateFormat.MEDIUM); DateFormat longDateFormat = DateFormat.getDateTimeInstance( DateFormat.LONG, DateFormat.LONG); DateFormat fullDateFormat = DateFormat.getDateTimeInstance( DateFormat.FULL, DateFormat.FULL); System.out.println(shortDateFormat.format(date)); System.out.println(mediumDateFormat.format(date)); System.out.println(longDateFormat.format(date)); System.out.println(fullDateFormat.format(date)); } } 注意我们在对 getDateTimeInstance的每次调用中都传递了两个值：第一个参数是日期风格，而第二个参数是时间风格。它们都是基本数据类型int(整型)。考虑到可读性，我们使用了DateFormat 类提供的常量: SHORT，MEDIUM，LONG，和 FULL。 import java.util.GregorianCalendar; import java.util.Date; import java.text.DateFormat; public class DateExample5 { public static void main(String[] args) { DateFormat dateFormat = DateFormat.getDateInstance(DateFormat.FULL); // Create our Gregorian Calendar. GregorianCalendar cal = new GregorianCalendar(); // Set the date and time of our calendar // to the system′s date and time cal.setTime(new Date()); System.out.println("System Date: " + dateFormat.format(cal.getTime())); // Set the day of week to FRIDAY cal.set(GregorianCalendar.DAY_OF_WEEK, GregorianCalendar.FRIDAY); System.out.println("After Setting Day of Week to Friday: " +dateFormat.format(cal.getTime())); int friday13Counter = 0; while (friday13Counter <= 10) { // Go to the next Friday by adding 7 days. cal.add(GregorianCalendar.DAY_OF_MONTH，7); // If the day of month is 13 we have // another Friday the 13th. if (cal.get(GregorianCalendar.DAY_OF_MONTH) == 13) { friday13Counter++; System.out.println(dateFormat.format(cal.getTime())); } } } }