Re-stepping Learning Java _DAY14 (regular expression, Math,random,system,biginteger,bigdecimal,date,calendar)

1: Regular Expressions (understanding)
(1) A string that conforms to a certain rule
(2) Common rules
A: Character
x character X. Example: ' A ' denotes character a
\ \ backslash character.
\ n New Line (newline) character (' \u000a ')
\ r return character (' \u000d ')

B: Character class
[ABC] A, B or C (simple Class)
[^ABC] Any character except A, B, or C (negation)
[A-za-z] A to Z or A to Z, the letters at both ends are included (range)
[0-9] 0 to 9 characters are included

C: Predefined character classes
. Any character. And mine is. The character itself, how to express it? \.
\d number: [0-9]
\w Word character: [a-za-z_0-9]
Things that make up words in regular expressions must have these things.

D: Boundary Matching device
^ The beginning of the line
End of the $ line
\b Word boundaries
is the place where the word character is not.
Example: Hello World?haha;xixi

E:greedy number of words
X? X, not once or once
X* X, 0 or more times
x+ X, one or more times
X{n} X, exactly n times
X{n,} X, at least n times
X{n,m} X, at least n times, but no more than m times
(3) Common function: (who is it used separately?)
A: Judging function
Public boolean matches of the String class (string regex)
B: Split function
Public string[of the String class] split (string regex)
C: Replace function
public string ReplaceAll of the String class (String regex,string replacement)
D: Get Features
Pattern and Matcher
Pattern p = pattern.compile ("A*b");
Matcher m = P.matcher ("Aaaaab");

Find (): Lookup does not exist
Group (): Get the data you just looked up
(4) Case
A: Determine phone number and mailbox
B: Split data according to different rules
C: Replace the number in the forum with *
D: Get a 3-character word in a string

2:math (Master)
(1) classes that operate on mathematical operations
(2) Common methods (self-completion)
A: Absolute Value
B: Rounding up
C: Rounding Down
D: Large values in two data
Power of E:a B
F: Random number
G: Rounding
H: Positive square root
(3) Case:
A: Guess the number games
B: Get random numbers of any range

3:random (understanding)
(1) A class for generating random numbers
(2) Construction method:
A:random () default seed, different random number generated each time
B:random (long Seed) specifies the seed, and the random number is the same each time the seed is the same
(3) member method:
A:int Nextint () returns the random number in the INT range
B:int nextint (int n) returns the random number in the range of [0,n]

4:system (Master)
(1) System classes, which provide some useful fields and methods
(2) member method (self-completion)
A: Run the garbage collector
B: Exit the JVM
C: Gets the millisecond value of the current time
D: Array Replication

5:biginteger (understanding)
(1) Operations on large integers
(2) Construction method
A:biginteger (String s)
(3) member method (self-completion)
A: Plus
B: Minus
C: Multiply
D: Except
E: Quotient and remainder

6:bigdecimal (understanding)
(1) floating point data to do operations, will lose precision. Therefore, it is recommended to use BigDecimal for floating-point data operations. (Financial-related projects)
(2) Construction method
A:bigdecimal (String s)
(3) member method:
A: Plus
B: Minus
C: Multiply
D: Except
E: Keep a few decimals yourself

7:date/dateformat (Master)
(1) Date is a class of dates that can be accurate to milliseconds.
A: Construction method
Date ()
Date (long time)
B: Member Method
GetTime ()
SetTime (long time)
C: Conversion of date and millisecond values to each other
Case: How many days have you been in this world?
(2) DateFormat classes that are formatted for dates and parsed for strings, but are abstract classes, use their subclasses SimpleDateFormat
A:simpledateformat (String pattern) given pattern
YYYY-MM-DD HH:mm:ss
B: Conversion of dates and strings
A:date--String
Format ()

B:string--Date
Parse ()
C: Case:
A tool class for date manipulation was created.

8:calendar (Master)
(1) The Calendar class, which encapsulates all the Calendar field values, can be obtained using a uniform method based on the incoming different calendar fields.
(2) How do I get a calendar object?
Calendar RightNow = Calendar.getinstance ();
Essentially, the subclass object is returned.
(3) member method
A: Get the corresponding value according to the Calendar field
B: Determine whether to add or subtract the value of the corresponding Calendar field based on the Calendar field and a positive negative number
C: Set Calendar object's Month day
(4) Case:
Calculate how many days in February of any year?

Re-stepping Learning Java _DAY14 (regular expression, Math,random,system,biginteger,bigdecimal,date,calendar)