The use of Java date and time related classes

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The Date class is used to represent time and date. Since internationalization was not considered during development, the Calendar class and DateFormat class were newly designed later .

One, Date class

1.Date class overview

• The Date class represents a specific instant, accurate to the millisecond.
• The Date class is passedOrigin of time( 1970.01.01 00:00:00:000) AndMillisecond value( long类型) To calculate time.
• The Date class comes from the java.util package, and the package needs to be imported when using it.

2. Common methods of Date class

Construction method

public Date()
Allocate a Date object and initialize this object to indicate the time (accurate to the millisecond) when it was allocated.

//打印当前系统日期
Date d = new Date();
System.out.println(d);

public Date(long date)
A Date object is allocated and initialized to represent the specified number of milliseconds since the standard base time (called the "epoch", which is 00:00:00 GMT on January 1, 1970).

//打印从标准基准时间以后的指定毫秒数的对应时间。
Date d = new Date(0L);
System.out.println(d);

Member method

long getTime()
Returns the number of milliseconds represented by this Date object since January 1, 1970 00:00:00 GMT.

//打印从基准时间到当前时间的毫秒数
Date d = new Date();
System.out.println(d.getTime());

Second, the Calendar class

1. Overview of the Calendar class

• The Calendar class is an abstract class.
• Provides fields related to the calendar.
• The getInstance method of Calendar returns a Calendar object whose calendar fields have been initialized with the current date and time.

2.Calendar commonly used methods

Let's take a look at the commonly usedCalendar field constant

Member method

int get(int field)
Parameters: Pass a specific calendar field.
Return value: Returns the value of the given calendar field.

//使用getInstance()创建对象
Calendar c = Calendar.getInstance();
//获取当前年份
int year = c.get(Calendar.YEAR);
//获取当前月份（西方月份从 0 开始，因此需要 + 1）
int month = c.get(Calendar.MONTH) + 1;
//获取当前日期
int date = c.get(Calendar.DATE);

System.out.println(year+"年"+month+"月"+date+"日");

void set(int field, int value)
Parameters: Pass the calendar field.
           Pass a value to the specified field

//使用getInstance()创建对象
Calendar c = Calendar.getInstance();
//单独设置年月日
c.set(Calendar.YEAR,2000);
c.set(Calendar.MONTH,02);
c.set(Calendar.DATE,20);
//可以同时设置 年、月、日
c.set(2200,03,05);
//设置完之后可用get方法取出，不在演示

abstract void add(int field, int amount)
Parameters: Pass the calendar field.
           Increase or decrease the value (positive numbers increase, negative numbers decrease)

//使用getInstance()创建对象
Calendar c = Calendar.getInstance();
//减少指定年份
c.add(Calendar.YEAR,-3);
//增加指定月份
c.add(Calendar.MONTH,3);

Date getTime()
Returns a Date object representing the time value of this Calendar (the millisecond offset from the epoch to the present).

//创建日历对象
Calendar c = Calendar.getInstance();
//调用getTime方法
Date date = c.getTime();
System.out.println(date);

Three, DateFormat class

1. Overview of the DateFormat class

• The DateFormat class is an abstract class of date/time formatting subclasses (必须使用它的子类SimpleDateFormat类 来创建对象）。
• The DateFormat class can 完成日期和文本之间的转换.
• The DateFarmat class comes from the java.text package, and the package needs to be imported when using it.

2. Date and time format

A little understanding, there will be specific use in the following code

3.[Simple]DateFormat class commonly used methods

SimpleDateFormat construction method

SimpleDateFormat(String pattern)
Construct SimpleDateFormat with the given pattern and the date format symbols of the default locale.

//规定需要转换的日期格式
SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 HH时mm分ss秒");

Member methods of DateFormat

String format(Date date)
Format a Date as a date/time string.

//规定需要转换的日期格式
SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 HH时mm分ss秒");
//获取当前日期
Date d = new Date();
//日期转文本
String str = sdf.format(d);
System.out.println(str);

Date parse(String source)
Parse the text from the beginning of the given string to generate a date .

//规定需要转换的日期格式
SimpleDateFormat sdf = new SimpleDateFormat("yyyy年MM月dd日 HH时mm分ss秒");
//文本转日期
Date date = sdf.parse("2020年07月15日 19时04分40秒");
System.out.println(date);