July 28, 2022 Gu Yujia's study notes

Java Value passing and so-called reference passing

Essentially java As long as the value is passed , All assignment parameters are copies of values

The reference data type copies the reference address , The basic data type copies values , The instance object itself is not passed

Interview questions ： The result of the printout is ？

Output results ： 

Commonly used api（ Application program interface ）

JDK Provide some written classes , Methods can be called directly to solve the problem

Class method , On the macro level, it can become an interface .

Time related api

The time zone ： East eight

Beijing time. ：+8

  Time stamp （ The world is fixed ）： Get the timestamp , It can be converted into the specific time and date of our current location through timestamp .

Time stamp ： GMT 1970.1.1 00:00:00 To the end 2022.7.28 9:29:30 Millisecond count

1s = 1000ms , 1min = 60s , 1h = 60min , 1day =24h.

 Get the timestamp 
//  It can be converted into the specific time and date of our current location through timestamp 
System.out.println(System.currentTimeMillis());

 Get the current date and time of the system 
Date date = new Date();
System.out.println(date);

compareTo

 

When you return a negative number , It indicates that the caller time is before the parameter time .

When to return to 0 when , It indicates that the caller time is the same as the parameter time .

When a positive number is returned , Explain that the caller's time is after the stated time .

Calendar class ： date 、 Time

Calendar Is an abstract class , can new Cannot create object

initialization ;

Provides a set of pairs “ Specific date 、 Minutes and seconds 、 week ” And other information operation functions . You can manipulate information in different time zones .

JDK1.1 Version start , When processing time and date , System recommended Calendar class

Calendar than Date Much more powerful

Unless you want to obtain the time, minutes and seconds of month, day and year at one time , Otherwise, all use Calendar.

The time zone

ZoneId（ Get a place id）

 Date formatting （SimpleDateFormat） 
 format： format Date type , hold Date Type into String type 
      We want to show the data to the client .
 parse： hold String Type of time , Turn into Date type 
       The time transmitted from the client , It's usually String type , Store in database .

 

yyyy Represents the year ,yy The last two representatives of the year

MM Representative month

dd On behalf of the day

HH For hours （24 hourly ） hh For hours （12 hourly ）

mm On behalf of

ss On behalf of the second

SSS On behalf of ms

The methods of tool classes are static

New time class

Instant

LocalDate

LocalTime

DateTimeFormatter

 The duration of the Duration

LocalDate： Get current date 

 

  Execution results ：

Determine if it's a leap year

Local time LocalTime

Get day and time  LocalDateTime

Deal with the formatting of dates  DateTimeFormatter 

hold Instant Turn into Date

hold Date Turn into Instant 

 

hold Instant Turn into LocalDateTime 

hold LocalDateTime Turn into Instant 

hold Date Turn into LocalDateTime 

hold LocalDateTime Turn into Date

 Math Mathematics

random number random（） double

Rounding up  ceil（） double

Rounding down  floor（）double

rounding  round（）long

BigDecimal Statistics

Random classes Random

 

 Arrays—— Array tool class

Sort of array

  Array search

Array copy

 

Array comparison

System class —— System class

Objects（ Tool class ）JDK1.7

Judge whether it is empty

StringBuffer and StringBuilder—— A variable sequence of characters

and String There are essential differences

StringBuffer You cannot assign values directly with the equal sign

 

  Additional

Delete

 

Delete the specified location

 

Insert

 

reverse （ The interview will ask ）

 

String How to reverse ？

1.

 

 2.

 

StringBuffer yes （ Threads ） synchronous . Security , Low efficiency

StringBuilder It's asynchronous . unsafe , Efficient

 

Interview questions ： The difference between the three strings