1. Format string
The static format() method of type String is used to create a formatted string.
（ 1 ）format ( String fromat , Object ... args )
format : format string
args : Arguments in the format string referenced by format specifiers. If there are arguments other than format specifiers, these extra arguments are ignored. The number of this parameter is variable and can be 0.
（ 2 ）format ( Local l , String format , Object ... args )
l : The locale to apply during formatting. If l is null, no localization is done.
format : format string
args : Arguments in the format string referenced by format specifiers. If there are arguments other than format specifiers, these extra arguments are ignored. The number of this parameter is variable and can be 0.
1.1 Date and time string formatting
In applications, it is often necessary to display the date and time. If you want to output a satisfactory date and time, you generally want to write a lot of code to achieve it. The format() method implements the formatting of date and time by giving a special conversion character as an argument.
1.1.1 Date formatting
Returns the number of days in a month
Date date = new Date(); //Create Date object date
String s = String.format("%te",date); //Format date by foramt() method
The value of s is the number of days in the current date. If today is the 1st, the value of s is 1.
| converter | illustrate | Example |
| %at | day of the month ( 1 ~ 31 ) | 2 |
| %tb | the short name of the month for the specified locale | Feb (English), February (Chinese) |
| %tB | the full name of the month for the specified locale | February (English), February (Chinese) |
| % tA | the full name of the day of the week for the specified locale | Monday (English), Monday (Chinese) |
| % ta | Abbreviated day of the week for the specified locale | Mon (English), Monday (Chinese) |
| %tc | Include full date and time information | Sunday April 01 21:33:22 CST 2018 |
| %tY | 4 digit year | 2018 |
| % i.e. | Day of the year ( 001 ~ 366 ) | 085 |
| %tm | month | 03 |
| %td | Day of the month ( 01 ~ 31 ) | 02 |
| %ty | 2-digit year | 08 |
import java.util.Date; //import package
public class Eval{ //Create class
public static void main(String[] args){ //Main method
Date date = new Date(); //Create a Date object
// format date
String year = String.format("%ty", date);
String month = String.format("%tB", date);
String day = String.format("%td", date);
// output information
System.out.println("This year is:" + year + "year");
System.out.println("Now is: " + month + "month");
System.out.println("Today is:" + day + "Number");
}
}
The running result is:
This year is: 2018 It is now: April Today is: 01
1.1.2 Time Formatting
The time can also be formatted using the format() method.
| converter | illustrate | Example |
| %tH | 2-digit 24-hour hour (00~23) | 14 |
| % tI | 2-digit 12-hour hour (01~12) | 05 |
| %tk | 2-digit 24-hour hour (0~23) | 5 |
| %tl | 2-digit 1 2-hour hour (1~12) | 10 |
| %tM | 2-digit minute (00~59) | 05 |
| %tS | 2-digit seconds (00~60) | 12 |
| %tL | 3-digit milliseconds (000~999) | 920 |
| %tN | 9-digit microseconds (000000000~999999999) | 062000000 |
| %tp | AM or PM marker in the specified locale | afternoon (Chinese), pm (English) |
| %tz | Numeric timezone offset relative to GMT RFC 82 format | +0800 |
| %tZ | a string of time zone abbreviations | CST |
| %ts | Number of seconds since 1970-01-01 00:00:00 | 1206426646 |
| % tQ | 1970-01-01 00:00:00 milliseconds since now | 1206426737453 |
import java.util.Date; //import package
public class GetDate{ //Create class
public static void mian(String[] args){ //main method
Date date = new Date(); //Create a Date object
//Format the date
String hour = new String.format("%tH",date);
String minute= new String.format("%tM",date);
String second = new String.format("%tS",date);
// output information
System,out,println("Now is:"+huor+"hour"+minute+"minute"+second+"second")
}
}
The result of the operation is: It is now 21:57:50
1.3 Formatting common date-time combinations
Formatted date-to-time converters define the format of various date-time combinations.
| converter | illustrate | Example |
| %tF | "Year-Month-Day" format (4-digit year) | 2018-04-01 |
| %tD | "month/day/year" format (2-digit year) | 04/01/18 |
| %tc | Full date and time information | Sunday April 01 22::04:00 CST 2018 |
| %tr | "Hour:Minute:Second PM ( AM )" format (12-hour clock) | 03:22:06 PM |
| %tT | "hours:minutes:seconds" format (24-hour clock) | 15:23:50 |
| %tR | "hour:minute" format (24-hour clock) | 15:25 |
import java.util.Date;
public class DateAndTime{
public static void main(String[] args){
Date date = new Date(); //Create a Date object
String time = String.format("%tc",date); //Format
String form = String.format("%tF",date);
System.out.println("All time information is: "+time); //Output information
System.out.println("年-月-日 格式:"+form);
}
}
运行结果为：
全部的时间信息是：星期日 四月 01 22:10:00 CST 2018
年-月-日 格式：2018-04-01
二、常规类型格式化
常规类型格式化可应用于任何参数类型。
| 转换符 | 说明 | 示例 |
| %b 、%B | 结果被格式化为布尔类型 | true |
| %h 、%H | 结果被格式化为散列码 | A05A5198 |
| %s 、%S | 结果被格式化为字符串类型 | " abcd " |
| %c 、%C | 结果被格式化为字符类型 | ' a ' |
| %d | 结果被格式化为十进制整数 | 40 |
| %o | 结果被格式化为八进制整数 | 11 |
| %x 、%X | 结果被格式化为十六进制整数 | 4b1 |
| %e | 结果被格式化为用计算机科学记数法表示的十进制数 | 1.700000e+01 |
| %a | 结果被格式化为带有效位数和指数的十六进制浮点值 | 0X1.c00000000001P4 |
| %n | 结果为特定于平台的行分隔符 | |
| %% | 结果为面值 ‘ % ’ | % |
public class General{
public static void main(String[] args){
String str = String.format("%d",400/2); //将结果以十进制格式显示
String str2 = String.format("%b",3>5); //将结果以 boolean 型显示
String str3 = String.format("%x",200); //将结果以十六进制格式显示
System.out.println("400 的一半是:"+str);
System.out.println("3 > 5 正确吗:"+str2);
System.out.println("200 的十六进制数是:"+str3);
}
}
运行结果为：
400 的一半是: 200 3 > 5 正确吗: false 200 的十六进制数是: c8
对于学习Java，看书，看视频，看官方文档，看API，多练，多想。
要时刻学习，更新很快，要学习新技术，好好加油，祝终成大神。