Date, time and date class. Here we talk about the util package, not the sql package

Date: indicates a specific time instant, accurate to milliseconds (1000 milliseconds = 1 second)

Time and date operations are based on milliseconds

Time origin: January 1, 1970, 0:00:00, 0:00, 0:00

Get the millisecond value of the current time:

public class DateDemo {
    public static void main(String[] args) {
        long l = System.currentTimeMillis();
        //Get the milliseconds value of the current date
        System.out.println(l);
        //1515639742854
    }
}

Construction method of Date class:

package demo;

import java.util.Date;

public class DateDemo {
    public static void main(String[] args) {
        function1();
        function2();
    }
    public static void function1(){
        Date date = new Date();
        System.out.println(date);
        //Output: Thu Jan 11 11:09:41 CST 2018
    }
    public static void function2(){
        Date date = new Date(123456);
        System.out.println(date);
        //Output: Thu Jan 01 08:02:03 CST 1970
    }
}

Method of Date class:

package demo;

import java.util.Date;

public class DateDemo {
    public static void main(String[] args) {
        function1();
        function2();
    }
    public static void function1(){
        //Date to MS
        Date date = new Date();
        long time = date.getTime();
        System.out.println(time);
        //1515642454307
    }
    public static void function2(){
        //Millisecond to date
        Date date = new Date();
        System.out.println(date);
        //Thu Jan 11 11:47:34 CST 2018
        
        date.setTime(123123);
        System.out.println(date);
        //Thu Jan 01 08:02:03 CST 1970
    }
}

We find that the Date format obtained by the Date class is not what we are used to,

So here is a DateFormat class that provides the function of formatting dates:

DateFormat is an abstract class, which mainly uses its subclass SimpleDateFormat class

Example:

package demo;

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

//Format date
public class SimpleDateFormatDemo {
    public static void main(String[] args) {
        function();
    }
    
    public static void function(){
        DateFormat date = new SimpleDateFormat("yyyy year MM month dd day HH spot mm Minute ss second");
        String date1 = date.format(new Date());
        System.out.println(date1);
        //Output: 12:01:30 on January 11, 2018
    }
}

You can also convert strings to dates:

Example:

package demo;

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

//Format date
public class SimpleDateFormatDemo {
    public static void main(String[] args) throws ParseException {
        function();
    }

    public static void function() throws ParseException {
        // There are exceptions here, which will be introduced later
        DateFormat date1 = new SimpleDateFormat("yyyy-MM-dd");
        // Must be a string in standard format
        Date date2 = date1.parse("2017-11-25");
        System.out.println(date2);
        // Output: Sat Nov 25 00:00:00 CST 2017
    }
}

In actual development, users will not be allowed to enter dates by themselves, because there is a risk of formatting errors

Usually make a date control for the user to choose, rather than let the user input