import java.util.Date; 

public class Demo01Date { 
    public static void main(String[] args) { 

    // Create a date object and put the current time 
    System.out.println(new Date()); // Tue Jan 20 14:37:35 CST 2022

    // Create a date object and convert the current millisecond value into a date object 
    System.out.println(new Date(0L)); // Thu Jan 01 08:00:00 CST 1970 
    } 
}

import java.util.Date;

public class Demo02Date {
    public static void main(String[] args) {
        demo();
    }

    /*
        long getTime() Convert the date to a millisecond value (equivalent to the System.currentTimeMillis() method)
          Returns the number of milliseconds represented by this Date object since January 1, 1970, 00:00:00 GMT.
     */
    private static void demo() {
        Date date = new Date();
        long time = date.getTime();
        System.out.println(time); //1642667329975
    }
}

import java.util.Date;

public class test {
    public static void main(String[] args) {
        Date d = new Date();
        System.out.println(d.toLocaleString());  // January 20, 2022 8:01:46 PM
    }
}

2, DateFormat class

1. Overview

import java.text.DateFormat; 
import java.text.SimpleDateFormat; 

public class Demo02SimpleDateFormat { 
    public static void main(String[] args) { 

        // The corresponding date format is as follows: 2022 ‐ 01 ‐ 20 15:06:38 
        DateFormat format = new SimpleDateFormat("yyyy‐MM‐dd HH:mm:ss"); 
    } 
}

4. Common methods

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

/*Convert String to Date object */

public class Demo04DateFormatMethod {
    public static void main(String[] args) throws ParseException {
        DateFormat df = new SimpleDateFormat("yyyy year MM month dd day");
        String str = "2022 January 20";
        Date date = df.parse(str);
        System.out.println(date); // Thu Jan 20 00:00:00 CST 2022
    }
}

3, Practice

practice:
Please use the date and time related API to calculate how many days a person has been born.

import java.util.Calendar; 

public class Demo06CalendarInit { 
    public static void main(String[] args) { 
        Calendar cal = Calendar.getInstance(); 
    }
}

3. Common methods

import java.util.Calendar;
import java.util.Date;

/*
    Calendar Common member methods of class:
        public int get(int field): Returns the value of the given calendar field.
        public void set(int field, int value): Sets the given calendar field to the given value.
        public abstract void add(int field, int amount): Adds or subtracts the specified amount of time for a given calendar field according to the rules of the calendar.
        public Date getTime(): Returns a Date object representing this Calendar time value (the millisecond offset from the epoch to the present).
    Parameters of member method:
        int field:The fields of Calendar class can be obtained by using the static member variable of Calendar class
            public static final int YEAR = 1;	year
            public static final int MONTH = 2;	month
            public static final int DATE = 5;	One day in the middle of the month
            public static final int DAY_OF_MONTH = 5;One day in the middle of the month
            public static final int HOUR = 10; 		Time
            public static final int MINUTE = 12; 	branch
            public static final int SECOND = 13;	second
 */
public class Demo02Calendar {
    public static void main(String[] args) {
        demo01();
        System.out.println("==============");
        demo02();
        System.out.println("==============");
        demo03();
        System.out.println("==============");
        demo04();
    }

    /*
        public Date getTime(): Returns a Date object representing this Calendar time value (the millisecond offset from the epoch to the present).
        Convert calendar object to date object
     */
    private static void demo04() {
        //Use the getInstance method to get the Calendar object
        Calendar c = Calendar.getInstance();

        Date date = c.getTime();
        System.out.println(date);  // Thu Jan 20 19:41:43 CST 2022
    }

    /*
        public abstract void add(int field, int amount): Adds or subtracts the specified amount of time for a given calendar field according to the rules of the calendar.
        Increases / decreases the specified field by the specified value
        Parameters:
            int field:Pass the specified calendar field (YEAR,MONTH...)
            int amount:Increases / decreases the specified value
                Positive: increase
                Negative: decrease
     */
    private static void demo03() {
        //Use the getInstance method to get the Calendar object
        Calendar c = Calendar.getInstance();

        //Increase the year by two years
        c.add(Calendar.YEAR,2);
        //Reduce the month by three months
        c.add(Calendar.MONTH,-3);


        int year = c.get(Calendar.YEAR);
        System.out.println(year);

        int month = c.get(Calendar.MONTH);
        System.out.println(month);//Month 0-11 in the West and 1-12 in the East

        //int date = c.get(Calendar.DAY_OF_MONTH);
        int date = c.get(Calendar.DATE);
        System.out.println(date);
    }

    /*
        public void set(int field, int value): Sets the given calendar field to the given value.
        Parameters:
            int field:Pass the specified calendar field (YEAR,MONTH...)
            int value:The value set for the specified field
     */
    private static void demo02() {
        //Use the getInstance method to get the Calendar object
        Calendar c = Calendar.getInstance();

        //Set year to 9999
        c.set(Calendar.YEAR,9999);
        //Set month to September
        c.set(Calendar.MONTH,9);
        //Set day 9
        c.set(Calendar.DATE,9);

        //At the same time, set the month, year and day. You can use the overload method of set
        c.set(8888,8,8);

        int year = c.get(Calendar.YEAR);
        System.out.println(year);

        int month = c.get(Calendar.MONTH);
        System.out.println(month);//Month 0-11 in the West and 1-12 in the East

        int date = c.get(Calendar.DATE);
        System.out.println(date);
    }

    /*
        public int get(int field): Returns the value of the given calendar field.
        Parameter: pass the specified calendar field (YEAR,MONTH...)
        Return value: the specific value represented by the calendar field
     */
    private static void demo01() {
        //Use the getInstance method to get the Calendar object
        Calendar c = Calendar.getInstance();
        int year = c.get(c.YEAR);
        System.out.println(year);

        int month = c.get(Calendar.MONTH);
        System.out.println(month);//Month 0-11 in the West and 1-12 in the East

        //int date = c.get(Calendar.DAY_OF_MONTH);
        int date = c.get(Calendar.DATE);
        System.out.println(date);
    }
}