Javase | Date class, SimpleDateFormat class, System class

1.Date class (date class)

1.1 Construction method of Date class

public Date( )

• public Date( ): No-argument constructor to create a Date object .

• The output Date object is the current date and time of the system .

     //创建Date对象
     Date nowTime = new Date(); 
  
      //输出系统当前的“日期和时间”
     //Date类中的toString()方法被重写了，所以此处输出的不是一个内存地址，应是一个日期字符串。
     System.out.println(nowTime); //Thu Sep 14 14:36:24 CST 2023
  

public Date( long date )

• public Date( long date ): Construction method with parameters , passed" specified number of milliseconds "to create a Date object .

     //有参构造方法，通过“指定的毫秒数”创建Date对象
  
     /*
     这个时间是: 1970-01-01 00:00:00 001 (格林尼治时间)，
     通过下面的时间格式转换，可以验证
     */
     Date date = new Date(1); //注意: 参数 1 是一个毫秒数
  
     SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
     String format = sdf.format(date);
     System.out.println(format); //1970-01-01 08:00:00 001 (北京时间)
  

2.SimpleDateFormat class (simple date format class)

2.1 Definition of SimpleDateFormat class

• SimpleDateFormat( Simple date format class ): Under the Java . text package, specificallyResponsible for date formatting。

2.2 Construction method of SimpleDateFormat

SimpleDateFormat( )

• SimpleDateFormat( ): No-argument constructor to create a SimpleDateFormat object.

   //SimpleDateFormat 在Java.text包下的，专门负责日期格式化。
   SimpleDateFormat sdf = new SimpleDateFormat();

SimpleDateFormat( String pattern )

• SimpleDateFormat( String pattern ): Construction method with parameters , usegiven patternand default locale date format symbolConstruct SimpleDateFormat object

• in this constructorparameter: Date format symbol . That is, the content passed in the parameter is: date format symbol , to create a "simple date format" object

• SimpleDateFormat can be combined with.format(Date date)The method can get the " date information " in the specified format . The format() method is the method of DateFormat, the parent class of SimpleDateFormat.
  

   	     //创建Date对象
          Date nowTime = new Date();
          /*
             日期格式符号:
             yyyy 年
             MM   月
             dd   日
             HH   时
             mm   分
             ss   秒
             SSS  毫秒
  
            完整表示年月日时间秒的日期格式符号 : yyyy-MM-dd HH:mm:ss:SSS
           */
          //该构造方法中的参数为“日期格式符号”，以指定的日期格式来创建简单日期格式对象
  SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SSS");
          SimpleDateFormat sdf3 = new SimpleDateFormat("dd/MM/yyyy");
          SimpleDateFormat sdf4 = new SimpleDateFormat("yy/MM/dd HH:mm:ss");
  
          // 调用.format()方法 输出指定格式的"时间信息"
          String format2 = sdf2.format(nowTime);
          String format3 = sdf3.format(nowTime);
          String format4 = sdf4.format(nowTime);
  
          System.out.println(format2); //2023-09-13 20:24:48:243
          System.out.println(format3); //13/09/2023
          System.out.println(format4); //23/09/13 20:24:48
  

date format symbol

• date format symbolThe most commonly used ones are:

  yyyy year
  MM month
  dd day
  HH
  hour mm
  minute ss second
  SSS millisecond (highest three digits, up to 999, 1000 milliseconds represents 1 second)

• Date format symbol that fully represents year, month, day, time and seconds : yyyy-MM-dd HH:mm:ss:SSS

letter	date or time element	express	Example
G	Era identifier	Text	AD
y	Year	Year	1996; 96
M	month of year	Month	July; Jul; 07
w	week number in year	Number	27
W	week number in month	Number	2
D	days in year	Number	189
d	number of days in month	Number	10
F	week of month	Number	2
E	Days of the week	Text	Tuesday;Tue
a	Am/pm mark	Text	PM
H	Hour of day (0-23)	Number	0
k	Hour of day (1-24)	Number	24
K	Hour in am/pm (0-11)	Number	0
h	Hour in am/pm (1-12)	Number	12
m	minutes in hour	Number	30
s	seconds in minutes	Number	55
S	milliseconds	Number	978
z	Time zone	General time zone	Pacific Standard Time; PST; GMT-08:00
Z	Time zone	RFC 822 time zone	-0800

3. Convert "Date type" to "date string String"

• first step:Creates a SimpleDateFormat (date format notation) object.

• Step 2: Call SimpleDateFormat parent class: DateFormat method:format( Date date), to convert Date into " date string String " ( " date information " in the specified format ).

• important point:

  ① The parameters in new SimpleDateFormat (date format symbol) are:date format symbol。

  ② The parameters in the format(Date date) method are:Date object。

• Examples include:

  	    //创建Date对象
          Date date = new Date();
          /*
             日期格式符号:
             yyyy 年
             MM   月
             dd   日
             HH   时
             mm   分
             ss   秒
             SSS  毫秒
  
            完整表示年月日时间秒的日期格式符号 : yyyy-MM-dd HH:mm:ss:SSS
           */
          //该构造方法中的参数为“日期格式符号”，以指定的日期格式来创建简单日期格式对象
  SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SSS");
          SimpleDateFormat sdf3 = new SimpleDateFormat("dd/MM/yyyy");
          SimpleDateFormat sdf4 = new SimpleDateFormat("yy/MM/dd HH:mm:ss");
  
          // 调用.format()方法 输出指定格式的"时间信息"
          String format2 = sdf2.format(date);
          String format3 = sdf3.format(date);
          String format4 = sdf4.format(date);
  
          System.out.println(format2); //2023-09-13 20:24:48:243
          System.out.println(format3); //13/09/2023
          System.out.println(format4); //23/09/13 20:24:48
  

4. Convert "date string String" to "Date type"

• first step:Creates a SimpleDateFormat (date format notation) object.

• Step 2: Call SimpleDateFormat parent class: DateFormat method:parse( String pattern ), to convert " date string String " to " Date type ".

• important point:

  ① The parameters in new SimpleDateFormat (date format symbol) are:date format symbol. But the " date format symbol" Don't write casually, must be in the same format as "date string " , otherwise an exception will occur.

  ② The parameters in the parse(String pattern) method are:Date format string String。

• Examples include:

     //假设现在有一个日期字符串String，怎么转换为Date类型？
     String time = "2008-08-08 08:08:08 888";
     SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
     
     //调用parse()方法将"日期字符串String"转换为Date类型
     Date date = sdf.parse(time);
     System.out.println(date); //Fri Aug 08 08:08:08 CST 2008
  

5. Actual functional requirements:

Requirement: How to get the total number of milliseconds from 00:00:00 000 on January 1, 1970 to the "current system time"

   //获取自1970年1月1日 00:00:00 000到当前系统时间的总毫秒数。
   //1秒 = 1000毫秒
   long nowTimeMillis = System.currentTimeMillis();
   System.out.println(nowTimeMillis); //1694666718586

Requirements: Count the time it takes to execute a method

public class DateTest {
    
    
    public static void main(String[] args) {
    
    


        //统计一个方法执行所耗费的时长
        //在调用目标方法之前记录一个毫秒数
        long begin = System.currentTimeMillis();
        print();
        //在执行完目标方法之后记录一个毫秒数
        long end = System.currentTimeMillis();
        System.out.println("耗费的时长"+(end - begin)+"毫秒");
    }

    //统计一个方法执行所耗费的时长
    private static void print() {
    
    
        for (int i = 0; i < 1000; i++) {
    
    
            System.out.println("i= " + i);
        }
    }
}

Requirement: Get the time of "yesterday" at this time

   //获得"昨天"的此时的时间
   Date date = new Date(System.currentTimeMillis() - 1000 * 60 * 60 * 24);
   SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
   String strTime = sdf.format(date);
   System.out.println(strTime);

6.System class

Related properties and methods of the System class:

System.outout is a static variable of the System class .

System.out.println( ): The println method is not System's, but a method of PrintStream class .

System.gc( ): Start the garbage collection mechanism .

System.currentTimeMillis( ): Get the total number of milliseconds from 00:00:00 000 on January 1, 1970 to the current system time .

System.exit(0 ): Exit the JVM .