STRING AND STRING BUFFER

STRING AND STRING BUFFER

String class is a immutable class . Immutable classes are those classes in which if we create object of that class and after that alter the content of that object then every time a new object is created that store the altered content 

String buffer class is mutable class . Mutable classes are those classes in which if we create object of that class and after that alter the content of that object that changes will occur in same object . No new object is created .

EXAMPLE :

String s = new String("Hello");

s=s+"World"; 

Understand that , a string s is created in line 1 in which initially we put "Hello" in string s . And in statement second we have concatenated string s . And add further "World " to this .
As this is a string class and string classes are immutable . Hence , world will not be stored in same object where hello was stored . A new object will be created every time we concatenate the string or add anything in the string.

Now , consider the following set code :

StringBuffer b = new StringBuffer("Hello");

b.append("World");

Here , in the first line , string buffer is declared and in the object Hello is stored . When second line is read , to add more contents to string s we use keyword append . It does not  create a new object. World is added  to the existing object which contains Hello . 

NOTE :
If content of string  changes rapidly then instead of String , String buffer is convenient and is suggested .

Methods in CLASS BUFFER:

Consider a String defined this way :

   String s="Hello World";

We have a list of methods :

• s.length();
• s.charAt(n);
• s.toLowerCase();
• s.toUpperCase();
• s.indexOf('char');
• s.lastIndexOf('char');
• s.replace('x','y');
• s.subString(x);
• s.subString(x,y);
• s.equals("xyz");
• s.equalsIgnoreCase("xyz");

Understanding the above mentioned methods :

 s.length():
It will display out the characters used in the string . To print the number of characters on screen :
              System.out.println(s.length(););
And for above string s it would be 11 . Note that space is also considered to be a character.

s.charAt(n);

This methods gives the character present at nth position . Positions are considered by system in the following way :

Hello World
01234567891011
Hence ,
                            s.charAt(0) -- > H
                            s.charAt(1) -- > e
                            s.charAt(2) -- > l
                            s.charAt(3) -- > l
                            s.charAt(4) -- > 0
                            s.charAt(5) -- > ' '
                            s.charAt(6) -- > W
                            s.charAt(7) -- > o
                            s.charAt(8) -- > r
                            s.charAt(9) -- > l
                            s.charAt(10) -- > d


s.toLowerCase
This method changes the string s to Lower case i.e. all the characters in string s are changed to Lower case characters.

s.toUpperCase
This method changes all the characters of string s in Upper case characters i.e capital letters .

s.indexOf('l'):
This method gives the position or location of character l . Also in other words it is a kind of reverse of s.charAt method. 
Index of missing character is defined as -1.

s.replace('l','*');
It will replace the character l in the string with the character * . Generally .
In s.replace('x','y') , character x will be replaced by character y.

s.subString(6);
It will display all the character from position6 till the end of string . In other words , substring of characters from character 6 till end would be accessed.

s.subString(3,6);
If a single argument is passed through substring it will display the characters from 6 till end.In case of two arguments it will check from 3 to 6 .

s.equals("xyz");
It checks weather string in brackets i.e string xyz is same as string s or not . IT IS CASE SENSITIVE .

s.equalsIgnoreCase("xyz");
It is not case sensitive. This is only difference in s.equals and this method.

String s[] = s.split("x");
This method is used to split the string from character x.
If we split the string s from l , then:
   String s[]=s.split("l");
Four strings would be formed .

He
""               // an empty string as l appears two consecutive times
o Wor
d