Java class before the date of (1) -JDK8 date API

1. java.lang.System class
   public static long currentTimeMillis System class provides () returns the current time at 0:00 on January 1, 1970 0 seconds milliseconds time difference.
   This method is adapted to calculate the time difference.

public class Test {
    public static void main(String[] args) {
        System.out.println(System.currentTimeMillis());
    }
}

2. java.util.Date class
   represents a specific moment, with millisecond precision
    Constructor:

• Date (): use the Object constructor to create a no-argument can obtain the current local time.
• Date (long date)
  commonly used methods:

1. getTime (): returns since January 1, 1970 00:00:00 GMT This Date object
   number of milliseconds.
2. toString (): put this Date object is converted to the form of String: dow Mon dd
   HH: mm: SS ZZZ YYYY wherein: dow week in a day (the Sun, Mon, Tue,
   Wed, Thu, Fri, Sat), zzz is the time standard.
    Many other methods are outdated.

public static void test2(){
    //构造器一 date() 创建了一个对应当前时间的Date()对象
    Date date1=new Date();
    System.out.println(date1.toString());//Fri Feb 21 22:59:14 CST 2020
    System.out.println(date1.getTime());

    //构造器二  创建指定毫秒数的date对象
    Date date2 = new Date(1582298162369L);
    System.out.println(date2.toString());
     
     //在用Date设置一个时间是会发生偏移量 并且它的月份是从0开始的
     Date date1=new Date(2020,9,8);//Fri Oct 08 00:00:00 CST 3920
     //所以要想获得真确时间必须在年份上减去一个1900并在月份上减去1
     Date date1=new Date(2020-1900,9-1,8);//当然这个方法也早已经过时
     System.out.println(date1);
    }

3. java.sql.Date class

Corresponds to the date of the database type variable

//创建java.sql.date对象
java.sql.Date date3 = new java.sql.Date(1582298478304L);
System.out.println(date3);//只显示年月日
//如何将java.util.Date对象转换为sql.Date对象
//情况一 :强转
Date date4=new java.sql.Date(1582299029103L);
java.sql.Date date5=(java.sql.Date)date4;
//情况二
Date date6=new Date();
java.sql.Date date7=new java.sql.Date(date6.getTime());

4. java.text.SimpleDateFormat类

Date class international API not easily, largely abandoned, the java.text.SimpleDateFormat
class is not associated with a locale-specific manner to format and parsed class date.
It allows for formatting : Date -----> text, analytical: text -----> Date
 Format:

• SimpleDateFormat (): the default mode and locale create objects
• public SimpleDateFormat (String pattern): This constructor parameter pattern can be used
  to create an object of a specified format, which calls:
  public String format (a Date DATE): Formatting Object Time DATE
  l resolved:
  public a Date the parse (String Source): from a given start parsing the text string to generate
  a date.

 public static void test() throws ParseException {
      //实例化 使用默认构造器
      SimpleDateFormat sdf = new SimpleDateFormat();
      //格式化 日期--->字符串
      Date date = new Date();
//      System.out.println(date);
      String format=sdf.format(date);
      System.out.println(format);
      //解析
	String str="2020/2/22 下午1:14";//默认格式
	Date parse = sdf.parse(str);
	System.out.println(parse);
	}

If you want to format the specified way can be called with argument constructor

public static void test() throws ParseException {
 Date date = new Date();
 SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
    String format2 = sdf2.format(date);
    System.out.println(format2);//2020-02-22 01:23:15
    //将得到的字符串进行解析
    Date date1 = sdf2.parse(format2);
    System.out.println(date1);
}

date format specified herein will yyyy-MM-dd hh: ss printed parsing parses the same string can pass this format: mm

5. java.util.Calendar (Calendar) class

Calendar is an abstract base class, for performing primary functions between each operation date fields.
 acquisition method Calendar instance:

1. Use Calendar.getInstance () method
2. Call it a subclass of GregorianCalendar constructor.

Calendar instance of a system of abstract representation of the time, wanted to get through get (int field) method
time information want. For example YEAR, MONTH, DAY_OF_WEEK, HOUR_OF_DAY,
MINUTE, SECOND,

static  void test(){
    //初始化
    //方式一:创建其子类 GregorianCalendar对象
    //方式二调用静态方法
    Calendar instance = Calendar.getInstance();
    //常用方法
    //get()
    int day = instance.get(Calendar.DAY_OF_MONTH);//当前时间在这个月是第几天
    System.out.println(day);
    System.out.println(instance.get(Calendar.DAY_OF_YEAR));
    
    //set()
    instance.set(Calendar.DAY_OF_MONTH,23);//修改了原本的instance值
    day = instance.get(Calendar.DAY_OF_MONTH);
    System.out.println(day);
    
    //add()
   instance.add(Calendar.DAY_OF_MONTH,3);//在原本的值上增加3天
   day = instance.get(Calendar.DAY_OF_MONTH);  
   System.out.println(day);
   
   //getTime():日历类---->Date
    Date time = instance.getTime();//返回一个当前时间的Date对象
    System.out.println(time);
    
    //setTime Date----->日历类
    Date date=new Date();
    instance.setTime(date);//Date转货为日历类
    day = instance.get(Calendar.DAY_OF_MONTH);//在获取这个date时间是当前月的第几天
    System.out.println(day);
}

Note: 

• Getting Month: January is 0, February is 1, and so on, December 11
• When acquiring a week: Sunday is 1, Tuesday is 2. . . . Saturday is 7