Java中关于时间日期的部分

1.SimpleDateFormat2.获取当前时间的毫秒值3.日期转字符串、字符串转日期4.输入时间，转为日期5.计算时间差

主要就是用来规范我们输出的时间日期格式

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); 

//Demo1：
Date date = new Date();  
System.out.println(date.getTime());  

//Demo2：
System.out.println(System.currentTimeMillis());  

//Demo3：
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); 
String current = sdf.format(System.currentTimeMillis());  
System.out.println(current); 

// An highlighted block
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
try {  
    //1、日期转字符串  
    Calendar calendar = Calendar.getInstance();  
    Date date = calendar.getTime();  
    String dateStringParse = sdf.format(date);  
    System.out.println(dateStringParse);  
    //2、字符串转日期  
    String dateString = "2022-02-22 22:22:22";  
    Date dateParse = sdf.parse(dateString);  
    System.out.println(dateParse);  
} catch (ParseException e) {  
    e.printStackTrace();  
} 

// An highlighted block
public static String getFormatBySeconds(long seconds) {      
        String timeStr = seconds + "秒";      
        if (seconds > 60) {      
            long second = seconds % 60;      
            long min = seconds / 60;      
            timeStr = min + "分" + second + "秒";      
            if (min > 60) {      
                min = (seconds / 60) % 60;      
                long hour = (seconds / 60) / 60;      
                timeStr = hour + "小时" + min + "分" + second + "秒";      
                if (hour > 24) {      
                    hour = ((seconds / 60) / 60) % 24;      
                    long day = (((seconds / 60) / 60) / 24);      
                    timeStr = day + "天" + hour + "小时" + min + "分" + second + "秒";      
                }      
            }      
        }      
        return timeStr;      
 }      

// An highlighted block
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

//最好从DB查
String previousTime = "2022-01-01 01:01:01";  
Calendar calender = Calendar.getInstance();  
long currentDate = calendar.getTime().getTime(); 
  
try {  
    long previousDate = sdf.parse(previousTime).getTime();  
    long betweenDate = (currentDate - previousDate) / (1000 * 60 * 60 * 24); 
    System.out.print(betweenDate);  
} catch (ParseException e) {  
    e.printStackTrace();  
}