String handling functions in java

String class is encapsulated in java.lang package. every string that u create is an actually object of type String .

String object is that Objects are immutable i.e means once string object is created  it can't be altered .

What is an Immutable Object ?

An object whose state can't be changed  after it is created is known as immutable object.String,Integer, like all wrapper classes .

creating immutable class :

public final String Test
{
 final String str;

 Test(String s)
{
this.str = s;
}
public String get()
{
 return str;
   }
}

Note : Test  is an immutable class  its state cannot be changed .

Creating a String Object is the following ways.

1) Using literal : enclosed with  " " (double quotes).

   String str1 = "Hello" ;

2) Using another String object :

  String str2 = new String (str1);

3) Using new Keyword :
    String str3 = new String ("JAVA");
    
4) Using + Operator (Concatenation)

    String str4 = str1+str2 ;
     or
   String str5 = "Hello" +"JAVA ";

Each time you creates a String pool(literal) , the jvm checks the string pool first . if the already exists in the pool . a reference to the pool instance is returned .
If String does not existing in the pool , new string object is created ,and placed in the pool . String objects are stored in special memory  area known as  String constantPool(string literal pool) in side the heap memory  .

 String str = "Hello" ;

And we created as  the same object with same string  ,then reference of the string literal  already present in string pool is returned .

 String str = str ;

But if we change  the new String  its reference should be modified .

  str2 = str2.concat("world");

Concatenating String :

 1 ) Using concat() method
 2 ) Use + operator

1) Using concat() method:

     String s = "Hello";
     String str = "JAVA";
     String str2 = s.concat(str);
     String str1 = "Hello".concat("JAVA"); //works with string literals too.

2) Using  + operator :

   String str1 = "RAHUL";
   String str2 = "DRAVID";
   String s3 = str1+str2;
   String st ="RAHUL"+"DRAVID";

String Comparison:

 1) Using equals() method 
 2) Using == operator
 3) By CompareTo() method

Using equals() method:

 equal method compares two strings for equality .

  Syntax  : boolean equals(Object str)

it compares the content of 2 strings  . it will return true if string matches  , else return false .

 String s = "Hello ";
 String s1 = "Hello";

 String s2 ="Hello";

 s1.equals(s2);//true
 s.equals(s1); // false

Using == operator:

String s1 ="Kavya";
String s2 ="Kavya";
String s3 = "Kavya";

test(s1==s2); //true
test(s1==s3) ; //false

By CompareTo() method:

Syntax  :   int compareTo(String str)

 To use this function you have to implement from Comparable interface . compareTo() the  only function in comparable interface .

 String s1 = "Sachin";
 String s2 = "Tendulkar";
 String s3 = "Sachin";

  s1.compareTo(s2); // returns  -1 becoz  s1<s2
  s1.compareTo(s3); // returns   0 becoz  s1==s3
  s2.compareTo(s1) ; //returns   1 becoz  s2>s1