Core classes involved: Date class, SimpleDateFormat class, Calendar class
1. Date type and long type
Date type is converted to long type
Date date = new Date();//Get the current time Date type
long date2long = date.getTime();//Date to long
long type converted to Date type
long cur = System.currentTimeMills();//Get the current time long type and return
Date long2date = new Date(cur); //long to Date
2. Date type and String type
Date type converted to String type
Date date = new Date();
SimpleDateFormat sdf = new SimpleDateFormat(“yyyy-MM-dd HH:mm:ss .SSS”);//Set the target conversion format to yyyy-MM-dd HH:mm:ss.SSS
String date2string = sdf.format(date);//Date to String
String type to Date type
String str="2001-11-03 11:12:33.828";//Set the initial string type date
Date str2date=sdf.parse(str);//Convert String to Date
3. Date type and Calendar type
Date type is converted to Calendar type
Calendar cal = Calendar.getInstance(); //Get the current time Calendar type
cal.setTime(date); //Date is converted to Calendar
Calendar type is converted to Date type
Calendar cal = Calendar.getInstance();//Get the current time Calendar type
Date cal2date = cal.getTime();//Convert Calendar to Date
4. Summary
Conversion between String and basic types relies on String.valueOf () method
The conversion between Date and String classes relies on the SimpleDateFormat class
The conversion between Date and long relies on the construct and getTime() method provided by Date
The conversion between Date and Calendar relies on the setTime() and getTime provided by Calendar () method
5. Interview questions
Q: Write a method, the parameter is Date date, push the date back 3 days, and return the string type in the "yyyy-mm-dd" format
public String add3Day(Date date) throws ParseException{ SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd"); Calendar cal = Calendar.getInstance(); cal.setTime(date);//Date转换为Calendar cal.add(Calendar.DATE, 3);//将日期往后推3天,减少3天则-3. 月增加则Calendar.MONTH String after = sdf.format(cal.getTime());//Calendar转换为Date,再转换为String return after; }