String is a class in Java, it has following methods:

SN	String Method	Method Usage	Return Type
1	length()	It returns the length of the String	int
2	charAt(int index)	It returns the character from the String at mentioned Index	char
3	toCharArray()	Converts the String into a char array	char[]
4	setCharAt(int index, char c)	Sets the character at the specified index of the String	void
5	toLowerCase()	Converts all the characters of the String into lower case	String
6	toUpperCase()	Converts all the characters of the String into upper case	String
7	isEmpty()	It returns true is the String is not null else false	boolean
8	substring(int start, int end)	It returns the String starting from index 'start' and ending at index 'end - 1'	String
9	substring(int start)	It returns the String starting at index 'start' until the last character of the String	String
10	equals()	It returns true if the Strings are same else false	boolean
11	compareTo(String s2)	It returns -1 if first String is lexicographically smaller than String s2 else it returns 1. It returns 0 in case if both the Strings are same lexicographically	int
12	replace(char a, char b)	It replace all the 'a' character occurrence with 'b'	String
13	replaceAll(String regular_expression, String replacement)	Replace all the character sequence matching 'regular_expression' with 'replacement'	String
14	replaceAll("//w", "")	It used for removing all the word characters from the String	String
15	replaceAll("//W", "")	It used for removing all the characters from the String that are not word characters	String
16	replaceAll("//s", "")	It used for removing all the white spaces from the String	String
17	replaceAll("[-+.^:,]","")	It used for removing all the special characters from the String	String
18	replaceAll("\\p{Punct}", "")	It used for removing all the punctuations from the String	String
19	replaceAll("\\p{Blank}","")	It used for removing all the spaces and tabs from the String	String
20	trim()	It removes the leading and trailing spaces from the String	void
21	split(String regular_expression)	Splits the String at every occurrence of specified regular expression	String[]
22	matches(String regular_expression)	It returns true if the String matches regular expression else false	boolean
23	indexOf(String s)	It returns the index of the given String 's'	int
24	indexOf(String s, int startIndex)	It returns the index of the given String when started from 'startIndex'	int
25	indexOf(char c)	It returns the index of the given character in the String	int
26	indexOf(char c, int startIndex)	It returns the index of the given character in the String when started from 'startIndex'	int
27	subSequence(int start, int end)	It returns character sequence from index 'start' to index 'end-1'	charSequence
28	contains(CharSequence char_set)	It returns true is the character sequence matches else false	boolean

StringBuilder is a class in Java, it creates sequence of characters that are mutable. It has following methods:

SN	StringBuilder Methods	Method Usage	Return Type
1	reverse()	It reverses the sequence of the characters	StringBuilder
2	append(char c)	Appends the character to this String	void
3	append(String s)	Appends the String to this String	void
4	toString()	It returns the String representation of StringBuilder object	String
5	length()	It returns the length of the sequence of characters	int
6	setCharAt(int index, char c)	It sets the character at the specified index and return a new String	StringBuilder
7	delete(int startIndex, int endIndex)	It returns the String Builder object after deleting the characters at the specified Index range	StringBuilder
8	deleteCharAt(int index)	It deletes the character at the specified Index from the sequence of characters	StringBuilder
9	replace(int startIndex, int endIndex, String s)	It replace the sequence of characters with String 's' in the specified Index range	StringBuilder

Regular Expressions in Java:

\d represents any digit in regular expression but \ is a backlash character in java so to convert \ into a literal we add another \ so that \d can be considered as literal in regular expression!

Check all these regular expressions on a Java program

SN	Java Regex String	Description
1	[A-Za-z]	It matches a single character with all the uppercase and lowercase characters
2	[A-Za-z]?	It matches zero or one character with all the uppercase and lowercase characters
3	[A-Za-z]*	It matches zero or more than zero characters with all the uppercase and lowercase characters
4	[^A-Za-z]+	It matches a single or more characters with everything except A-Za-z
5	[^A-Za-z]{2}	It matches exactly two characters with everything except A-Za-z
6	[A-Za-z0-9]	It matches all the uppercase and lowercase alphanumeric characters
7	[A-Za-z0-9]{1,5}	It matches minimum 1 and maximum 5 uppercase and lowercase alphanumeric characters
8	[A-Za-z !,?._'@]+	It matches 1 or more than 1 lowercase and uppercase alphabets along with special characters
9	[-+.^:,]	It matches a single character with all the special characters
10	.	It matches with a character of any kind
11	\\d{1}	It matches with a single digit
12	\\d{1,3}	It matches with a numerical value having 1 to 3 digits
13	[\\d]{1}	It matches with a single digit
14	[0-5]	It matches with a single digit ranging from 0 to 5
15	\\s	It matches with single white space character
16	[\\s]	It matches with single white space character

Implementations:

Taking the input from user:

Scanner in = new Scanner(System.in);
String s = in.next(); /* Spaces will not be tackled */
String s=in.nextLine(); /* The letters in the entire line will be considered including spaces */

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Getting the character for a particular Ascii value:

char character= (char) i;

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Getting the Ascii value for a particular character in a String:

char character = sb.charAt(i);
int ascii = (int) character;

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Matching with a regular expression:

String s = "*******@gmail.com"

Pattern p = Pattern.compile(".+@gmail\\.com$");

Matcher matcher = p.matcher(s);

if(matcher.find()){
    System.out.println("The Person has a Gmail account!");
}

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