String handling in java

String:

• String is nothing but sequence of characters
• An array of characters works same as java String

For Example:  "luckysir" is a String of 8 characters

• In java, String is immutable object  
• immutable means it is constant and can not be changed once it has been created.
• Once we created String objects we are not allowed to perform any changes in the existing object
• If you are trying to perform any changes with those changes a new string  object will be created this behavior is nothing but"immutable" of the string object

Creating a string:                                          

                                                                                      

There are two ways  to create a String in java:           

a)String literal

b)using new keyword

a) String literal:

java String literal is created by using double quotes.

Example:

String s1="lucky";

String s2="lucky";//will not create new object 

• In the above example, only one object will be created
• First time JVM will not find any string object with the value "lucky"in string constant pool
• so it will create a new object. After  that it will find a with the same value "lucky"in the pool
• It will not create a new object but will return reference to the same instance

Important Note:

String objects are stored in a special memory area known as string constant pool

b) using new keyword:

 Ex:  String s=new String("lucky");

• In the above example, two objects will be created one is on heap and another one is on string constant pool(SCP) and 's' is referring to heap object  

                                        

• object creation in SCP is optional
• If the object is not available then only new object will be created

Note:      

• The object present in the SCP is not eligible for garbage collection even though the object does not have any reference variable
• All the SCP objects will be destroyed at the time of JVM shutdown

Important methods of String class:

charAt():

  The java String charAt() method returns a char value at the given index number.

   the index number start from 0.

Syntax:

    public char charAt(int index);

    Ex:  String s="lucky";

            System.out.println(s.charAt(2));// returns the char value as'c'

            System.out.println(s.charAt(8));// StringIndexOutOfBoundException

Concat():

The java String Concat() method combines specified string at the end of this string.

It returns combined string. It is like appending another string

Syntax:

public String Concat(String s);

 Ex:   String s="lucky";

          s1=s.concat("sir");

          System.out.println(s1);// returns string as luckysir

equals():

The java string equals()method compares two given strings based on the content of the string.If any character is not matched it returns false,if all characters are matched it returns true.

Syntax:
public Boolean equals(Object o);

    


 Ex:   

public class EqualsTest{

public static void main(String args[]){

String s="luckysir";

String s1="luckysir";

String s3="LUCKYSIR";

String s4="java";

System.out.println(s1.equals(s2));//return true because content and case is same

System.out.println(s1.equals(s3));//return false  because case not the same

System.out.println(s1.equals(s4));//return false because content not the same

}

}

}

equalsIgnoreCase():

Syntax:

public Boolean equalsIgnoreCase(String s);

Ex:   String s="lucky";

         System.out.println(s.equalIgnoreCase("LUCKY")); // return True

Note:  Usually for validating username(where case sensitive not important method, we can use equlsIgnorecase) but for validating password we can use equals method where the case is important.

  

                

 length():

 The java string length()method  is used to find length of the string.

Syntax:

public int legnth(String s);

 Ex:    String s="lucky";

           System.out.println(s.length);//compile time error

           System.out.println(s.length());//5

NOTE:  length is the variable applicable for array object, length() is method applicable for string object

Replace():

The java string replace()method returns a string replacing old string to new string(charsequence or char)

Syntax:

public String replace(char old, char new);

Ex:

String s="ababa";

System.out.println(s.replace('a','b'));

Output:

 bbbbb

SubString():

The java string subsring() method returns a part of the string.

Syntax:

public String substring(int begin);

Ex 1:  
String s="lucky";

System.out.println(s.substring(3));// return ky

public String substring(int begin,int end);

Ex 2:    
String s="luckysir";

System.out.println(s.substring(2,5));//cky

toLowerCase():

The java string toLowerCase()method returns the string in lower case letters.

Syntax:

public String toLowerCase();

Ex:  String s="LUCKYSIR";

        String lower=s.toLowerCase();

        System.out.println(lower);

toUpperCase():

The java string toUpperCase()method returns string in uppercase letters.

Syntax:

public String toUpperCase();

Ex:  
String s="luckysir";

String upper=s.toUpperCase();
System.out.println(upper);

trim():

It removes blank spaces present at starting and ending of the string.

But not middle blanks.

Syntax:

public String trim();

Ex:
String s="  lucky sir";

System.out.println(s.trim());//lucky sir

indexOf():

It returns index of given character value or substring

if it is not found it returns -1.

Syntax:

 public int indexOf(char ch);

// returns first occurrence of index

 public int lastIndexOf(char ch);

 // returns last occurrence of index

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