1. Time and date

1.1 Date class (application)

• Overview of Date class

  Date represents a specific time, accurate to milliseconds

• Date class constructor

public class DateDemo01 {
    public static void main(String[] args) {
        //public Date(): allocate a Date object and initialize it so that it represents the time it is allocated, accurate to milliseconds
        Date d1 = new Date();
        System.out.println(d1);
        //public Date(long date): assign a Date object and initialize it to indicate that it starts from the standard base time
        Specified number of milliseconds
            long date = 1000*60*60;
        Date d2 = new Date(date);
        System.out.println(d2);
    }
}

1.2 common methods of date class (application)

• common method

import java.util.Date;
/*
    public Date(): Allocate a Date object and initialize it so that it represents the time it is allocated, accurate to milliseconds
    public Date(long date): Allocates a Date object and initializes it to represent the specified number of milliseconds from the standard base time
 */

/*
    public long getTime():Gets the millisecond value of the date object from 00:00:00, January 1, 1970 to the present
    public void setTime(long time):Set the time and give the value of milliseconds
 */

public class test {
    public static void main(String[] args) {
        //Create date object
        Date d = new Date();

        //public long getTime(): gets the millisecond value of the date object from 00:00:00 on January 1, 1970 to the present
        System.out.println(d.getTime()*1.0/1000/60/60/24/365+"year");

        //public void setTime(long time): sets the time. The value given is milliseconds
//        long time = 1000*60*60;
        long time = System.currentTimeMillis();
        d.setTime(time);

        System.out.println(d);
    }
}

1.3 SimpleDateFormat class (application)

• SimpleDateFormat class overview

  SimpleDateFormat is a concrete class for formatting and parsing dates in a locale sensitive manner.
  We focus on date formatting and parsing

• SimpleDateFormat class constructor

• Common methods of the SimpleDateFormat class

  • Format (from Date to String)

    • public final String format(Date date): formats the date into a date / time string

  • Parsing (from String to Date)

    • public Date parse(String source): parses text from the beginning of a given string to generate a date
      Sample code

package Heima.SimpleDateFormat.test1;

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

/*
    Construction method:
        public SimpleDateFormat​():Construct a SimpleDateFormat, using the default mode and date format
        public SimpleDateFormat​(String pattern):Construct a SimpleDateFormat using the given pattern and the default date format

    Format: from Date to String
        public final String format(Date date): Format the date as a date / time string

    Parsing: from String to Date
        public Date parse​(String source): Parses text from the beginning of a given string to generate a date
 */
public class SimpleDateFormatDemo {
    public static void main(String[] args) throws ParseException {
        //Format: from Date to String
        Date d = new Date();
//        SimpleDateFormat sdf = new SimpleDateFormat();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy year MM month dd day HH:mm:ss");

        String s = sdf.format(d);

        System.out.println(s);

        //Parse String---Date
        String ss = "2029-08-22 11:11:11";//ParseException
        SimpleDateFormat sdf2 =new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        final Date dd = sdf2.parse(ss);
        System.out.println(dd);
    }
}

Case: Date tool class

• Case requirements
  Define a date tool class (DateUtils), which contains two methods: convert the date to a string in the specified format; Parses a string into a specified format
  Then define a test class (DateDemo) to test the methods of the date tool class

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

public class DateUtils {
    //Object private ownership
    private DateUtils(){}

    /*
    Converts the date to a string in the specified format
    Return value type: String
    Parameters: date, date, string format
    */
    public static String  datetostring(Date d, String format){
        SimpleDateFormat sdf =new SimpleDateFormat(format);
        String d1 = sdf.format(d);
        return d1;
    }
    /*
    Parses the string to a date in the specified format
    Return value type: Date
    Parameters: String s, String format
    */
    public static Date stringtodate(String s,String format) throws ParseException {
        SimpleDateFormat dd = new SimpleDateFormat(format);
        Date p = dd.parse(s);
        return p;
    }
}

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

public class DateUtilsTest {
    public static void main(String[] args) throws ParseException {
        Date d = new Date();
        String format = "yyyy year MM month dd day HH Time mm branch ss second";

        //Date--String
        String datetostring = DateUtils.datetostring(d, format);
        System.out.println(datetostring);

        //String--Date
        String ss = "2029-08-22 11:11:11";
        String format2 = "yyyy-MM-dd HH:mm:ss";
        Date stringtodate = DateUtils.stringtodate(ss, format2);
        System.out.println(stringtodate);


    }
}

1.4Clalendar (calendar)

• Calendar Class overview
  Calendar provides some methods for the conversion between a specific moment and a set of calendar fields, and some methods for manipulating calendar fields
  Calendar provides a class method, getInstance, to get commonly useful objects of this type.
  This method returns a Calendar object.
  Its calendar field has been initialized with the current date and time: Calendar rightNow = Calendar.getInstance();

• Common methods of Calendar Class

import java.util.Calendar;

/*
    Calendar It provides some methods for the conversion between a specific moment and a set of calendar fields, and some methods for manipulating calendar fields

    Calendar A class method getInstance is provided to obtain commonly useful objects of this type. This method returns a Calendar object,
    Its calendar field has been initialized with the current date and time: Calendar rightNow = Calendar.getInstance();
 */
public class CalendarDemo {
    public static void main(String[] args) {
        //Get object
        Calendar c = Calendar.getInstance();//Polymorphic common objects
        System.out.println(c);

        //public int get(int filed)
        int year = c.get(Calendar.YEAR);
        int mouths = c.get(Calendar.MONTH)+1;
        int days = c.get(Calendar.DATE);
        System.out.println(year+"Year"+mouths+days);

    }
}

        //Demand 1: today 3 years ago
//        c.add(Calendar.YEAR,-3);
//        year = c.get(Calendar.YEAR);
//        month = c.get(Calendar.MONTH) + 1;
//        date = c.get(Calendar.DATE);
//        System.out.println(year + "year" + month + "month" + date + "day");

        //Demand 2: 10 days after 10 years
//        c.add(Calendar.YEAR,10);
//        c.add(Calendar.DATE,-10);
//        year = c.get(Calendar.YEAR);
//        month = c.get(Calendar.MONTH) + 1;
//        date = c.get(Calendar.DATE);
//        System.out.println(year + "year" + month + "month" + date + "day");

        //public final void set (int year,int month,int date): sets the month, year and day of the current calendar
        c.set(2050,10,10);
        year = c.get(Calendar.YEAR);
        month = c.get(Calendar.MONTH) + 1;
        date = c.get(Calendar.DATE);
        System.out.println(year + "year" + month + "month" + date + "day");

Case: February day

• Get the number of days in February of any year

/*
    Requirements:
        Get the number of days in February of any year

    Idea:
        1:Enter any year on the keyboard
        2:Set year, month and day of calendar object
            Year: from keyboard entry
            Month: set to March. The month starts from 0, so the set value is 2
            Day: set to 1 day
        3:3 March 1 is pushed forward one day, which is the last day of February
        4:Just get the output of this day
 */

import java.util.Calendar;
import java.util.Scanner;

public class ErYueTian {
    public static void main(String[] args) {
        //Enter any year on the keyboard
        Scanner sc = new Scanner(System.in);
        System.out.println("Please enter the year:");
        int year = sc.nextInt();

        //Set year, month and day of calendar object
        Calendar c = Calendar.getInstance();
        c.set(year,2,1);

        //March 1 is pushed forward one day, which is the last day of February
        c.add(Calendar.DATE,-1);

        //Just get the output of this day
        int i = c.get(Calendar.DATE);
        System.out.println(year+"In February"+i+"day");

    }
}