String class in java

In java String is a class and it is imutable so we can not change its value.In following code it will concate other string without any error.So I want to ask that if it is immutable then why in this following code value of String is changed??

import java.util.*;

public class conc
{
    public static void main(String args[])
    {
        String a="Sheetal";
        a=a+"Ga";
        System.out.println("Result:"+a);

    }
}

Answer

In the code that you have shown, you have not changed the original String object.

Instead, you have created a new String object, which represents a + "Ga", and then re-assigned it to the reference variable a.

Note that all variables in Java other than primitive types are references.