Java Learning Note 02

A string class of generic strings

• Declaring a string

>>string Str

• Create a string

The >>string (char a[]) method is used to create a character array as a String object

The >>string (char a[],int offset,int length) method is used to extract a portion of the character array A to create a string object, where the parameter offset represents the position at which the string is to be truncated, and length indicates how long to intercept the string

The >>string (char a[] value) method can be used to assign a new String object that represents the result of all object joins in the character array parameter

• Connection string

>> Use the "+" operator to implement the ability to concatenate multiple strings, or you can concatenate multiple operators and produce a string object

>> Use the "+" operator to connect to other base data types and convert other data types to strings

• Get string information

>> use the length () method of the string class to get the lengths of the declared string objects

• String Lookup

The >>indexof (string s) method is used to return the index position of the first occurrence of the argument string s in the specified string. When the method is executed, the position of S is searched from the beginning of the string, and if the string s is not retrieved, the return value of the method is-1

The >>lastindexof (String str) method is used to return the index position of the last occurrence of the specified string, which is the same as indexof ()

• Gets the character at the specified index position

The >>charat (int index) method returns the character of the specified index position to the

• Get substring

The >>substring (int beginindex) method is used to get the substring that is intercepted at the end of the string starting at the specified index position

The >>substring (int beginindex,int endIndex) method is used to get a substring that is intercepted at a specified position and ends at an index position

• Remove spaces

The >>trim () method returns a copy of the string, ignoring leading and trailing spaces

• String substitution

The >>replace (char Oldchar,char Newchar) method replaces the specified character or string with a new character or string, and returns the original string if there is no target string

• Judging the start and end of a string

The >>startswith (string prefix) method and the EndsWith (string suffix) method are used to determine whether a string starts or ends with the specified content, and the return value is of type Boolean

• Determine if strings are equal

>> "= =" is used to compare the addresses of two strings, even if the content length is exactly the same, the address of two strings may be different

The >>equals (string otherstr) method is used to compare the contents and length of two strings, returning true when identical

The >>equalsignorecase (string otherstr) method is used to compare the contents and length of two ignored case strings, returning true when identical

• Compare two strings in a dictionary order

The >>compareto (string otherstr) method compares two strings in a dictionary order. Starting from the first bit of the two strings, if the characters of the first bit of the two string are the same, the backward comparison continues until a different character is present. If the last one is the same (that is, the Equals (object) method returns a value of true), the result is 0, and if different, the string object precedes the argument string, the result of the comparison is a negative integer, and vice versa is a positive integer

• Letter size Conversion

The >>tolowercase () method is used to convert a string to lowercase

The >>touppercase () method is used to convert a string to uppercase

• String segmentation

The >>split (string sign) method splits the contents of a string by a specified split character or string and stores the split result in a string array

The >>split (String sign,int limit) method acts the same as above, but can limit the number of splits

Second, date and time string formatting

• Date formatting

commonly used date format translator

translator	Description	example
%te	one day in one months (1~31)	2
%tb	Specify the month abbreviation for the locale	Feb (English), February (Chinese)
%TB	Specifies the full month of the locale	February (English), February (Chinese)
%ta	Specifies the day of the week for the locale	Monday (English), Monday (Chinese)
%ta	Specifies the weekday abbreviation for the locale	Mon (English), Monday (Chinese)
%TC	includes all date and time information	Monday Two or Three month 13:37:22 CST.
%ty	4-bit year	to
%TJ	the day of the Year (001~366)	085
%tm	month	a
%td	the day of the one month (01~31)	Geneva
%ty	2-bit year	to

• Time formatting

time-formatted conversion characters

Conversion character	Description	Example
%th	2-digit 24 o'clock hour (00~23)	14
%ti	2-digit 12 o'clock hour (01~12)	05
%tk	2-digit 24 o'clock hour (0~23)	5
%tl	2-digit 12 o'clock hour (1~12)	10
%tm	2-digit minute (00~59)	05
%ts	Number of seconds in 2 digits (00~60)	12
%tl	Number of milliseconds in 3 digits (000~999)	920
%tn	9-digit subtle number (000000000~999999999)	062000000
%tp	Specify the morning or afternoon markers for the locale	Afternoon (Chinese), PM (English)
%tz	Digital time zone offset relative to GMT RFC 82 format	+0800
%tz	A string in the form of a time zone abbreviation	Cst
%ts	1970-01-01 00:00:00 to now the number of seconds elapsed	1206426646
%tq	1970-01-01 00:00:00 to now the number of milliseconds elapsed	1206412131231

• Formatting common date and time combinations

Common format for date and time combinations

Conversion character	Description	Example
%tf	"Year-month-day" format (4-bit year)	2008-03-25
%td	"Month/day/year" format (2-bit year)	03/25/08
%tc	All date and time information	Monday Two or Three 15:20:00 CST 2008
%tr	Time: minute: PM (AM) format (12 o'clock)	03:22:06 pm
%tt	"Hours: minutes: Seconds" format (24 o'clock)	15:23:50
%tr	"Time: Minute" format (24 o'clock)	15:25

Third, the general type of formatting

General translator

translator	Description	example
%b,%b	results are formatted as Boolean types	true
%h,%h	results are formatted as walk codes	a05a5198
%s,%s	result is formatted as a string type	"ABCD"
%c,%c	results are formatted as character types	' a '
%d	results are formatted as decimal integers	+
%o	results are formatted as octal integers	one
%x,%x	results are formatted as hexadecimal integers	4b1
% E	results are formatted as decimal numbers in computer science notation	1.700000e+01
% A	result is formatted as a hexadecimal floating-point value with significant digits and exponent	0x1.c000000000001p4
%n	result is a line delimiter specific to the platform
%	result is literal%	%

Four, the regular expression

>> regular expressions are often used to determine whether a statement satisfies a format. A regular expression is a string of characters with a special meaning that Fu Shi a meta-character called a regular expression.

metacharacters in regular expressions

Metacharacters	The notation in regular expressions	Significance
.	.	Represents any one character
\d	\\d	Represents any number of 0~9
\d	\\d	Represents any one non-numeric character
\s	\\s	Represents a blank character, such as ' \ t ', ' \ n '
\s	\\s	Represents non-whitespace characters
\w	\\w	Represents a character that can be used as an identifier, but does not include "$"
\w	\\w	Represents a character that cannot be used as an identifier
\p{lower}	\\p{lower}	Represents the lowercase letter A~z
\p{upper}	\\p{upper}	Represents the capital letter A~z
\P{ASCII}	\\P{ASCII}	ASCII characters
\p{alpha}	\\p{alpha}	Alphabetic characters
\p{digit}	\\p{digit}	decimal number, i.e. 0~9
\p{alnum}	\\p{alnum}	numeric or alphabetic characters
\P{PUNCT}	\\P{PUNCT}	Punctuation:! " #$%& ' () *+,-;:<=>[email protected][\]^_ ' {|} ~
\p{graph}	\\p{graph}	Visible characters: [\p{alnum}\p{punct}]
\p{print}	\\p{print}	printable characters: [\p{graph}\x20]
\p{blank}	\\p{blank}	Space or tab: [\ t]
\p{cntrl}	\\p{cntrl}	Control characters: [\x00-\x1f\x7f]

>> qualifier modifier for qualifying occurrences of metacharacters in regular expressions

Qualifier modifier

Qualifier modifier	Significance	Example
?	0 or 1 times	A?
*	0 or more times	A *
+	1 or more times	A +
N	Happens to happen n times	A{2}
{N,}	appears at least n times	A{3,}
{N,m}	Appears n~m times	a{2,6}

>> regular expression matching using the matches (String regex) method

Five, StringBuilder class

The >string class creates an object whose length is fixed and the content cannot be changed and compiled, although using "+" can achieve the purpose of attaching a new character or string, but "+" produces a new string instance that creates a new strings object in memory. To solve this problem, the system introduces the variable character sequence StringBuilder class, which greatly improves the efficiency of increasing the string frequently.

The >>append (content) method is used to append content to the string generator, enabling the acceptance of any type of data

The >>insert (int offset arg) method is used to insert data content into the string generator at the specified location

>>delete (int start,int end) method to remove a string that begins at a specified position to the end of a position

Java Learning Note 02