jdk 8之前的日期时间的API简单使用

①、Calendar日历类(抽象类）的使用

//1.实例化
//方式一：创建其子类（GregorianCalendar）的对象
//方式二：调用其静态方法getInstance()
Calendar calendar = Calendar.getInstance();
System.out.println(calendar.getClass()); //class java.util.GregorianCalendar

//2.常用方法
//get()
int days = calendar.get(Calendar.DAY_OF_MONTH);
System.out.println(days);
System.out.println(calendar.get(Calendar.DAY_OF_YEAR));

//set()
//calendar可变性
calendar.set(Calendar.DAY_OF_MONTH, 22);
days = calendar.get(Calendar.DAY_OF_MONTH);
System.out.println(days);

//add()
calendar.add(Calendar.DAY_OF_MONTH, -3);
days = calendar.get(Calendar.DAY_OF_MONTH);
System.out.println(days);

//getTime():日历类---> Date
Date date = calendar.getTime();
System.out.println(date);

//setTime():Date ---> 日历类
Date date1 = new Date();
calendar.setTime(date1);
days = calendar.get(Calendar.DAY_OF_MONTH);
System.out.println(days);

②、字符串"2020-09-08"转换为java.sql.Date

String birth = "2020-09-08";

SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd");
Date date = sdf1.parse(birth);
System.out.println(date); //Tue Sep 08 00:00:00 CST 2020
System.out.println(date.getTime());  //1599494400000

java.sql.Date birthDate = new java.sql.Date(date.getTime());
System.out.println(birthDate);  //2020-09-08

③、SimpleDateFormat对日期Date类的格式化和解析

//实例化SimpleDateFormat:使用默认的构造器
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：格式化的逆过程，字符串 ---> 日期
String str = "19-12-18 上午11:43";
Date date1 = sdf.parse(str);
System.out.println(date1);

//*************按照指定的方式格式化和解析：调用带参的构造器*****************
//        SimpleDateFormat sdf1 = new SimpleDateFormat("yyyyy.MMMMM.dd GGG hh:mm aaa");
SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");

//格式化
String format1 = sdf1.format(date);
System.out.println(format1);//2019-02-18 11:48:27
//解析:要求字符串必须是符合SimpleDateFormat识别的格式(通过构造器参数体现),
//否则，抛异常
Date date2 = sdf1.parse("2020-02-18 11:48:27");
System.out.println(date2);