How the String is immutable in JAVA? Explain with example.

Yes, Strings are immutable in java. Lets elaborate this with the help of some examples.

There are two ways to declare and define a String value in java

1.       String s = new String(“xyz”);

2.       String s = “xyz”;

In both the ways a new String object is created and is assigned with the value “xyz”. But in first way a single object is created in String pool and values “xyz” is refer to it and in second way two object are created as one in String pool and one in non string pool.

So far the string object is just behaving like a normal object but when we try to assign another value to same object created earlier referring some value like

String  s  =  “xyz”;

s =  s.concat(“abc”);

This will append the “abc” at the end of the “xyz” and assign it to s variable.

So what is immutability is all about which means we can’t assign another value to object.

In java the String objects are immutable and this immutability is achieved as

The VM(Java Virtual Machine)  create another object with the value “xyzabc”  and refer it the String variable s. The older value “xyz” still reside in the pool with the lost reference to s.

That’s why when we concat the string but not assign to to variable it will not added to String variable

s    = “xyz”;

s.concat(“abc”);

System.out.println(s);

It will still show the value “xyz”. To understand this more clearly please see the example below

Example Code:

public class demoString {

    public static void main(String s[])

    {

        String st = new String ("JAVA");

        st.concat(" is great"); //we created the new value JAVA is great but does not assign this to st     

                                                     variable

        System.out.println("The result of String : "+st);//this will only print JAVA

        String st1 = new String ("JAVA");

        st1 = st1.concat(" is great");//we created the new value JAVA is great and assign this to st1 variable

        System.out.println("The result of String : "+st1);//this will print JAVA is great

    }

}

Output :-

The result of String : JAVA

The result of String : JAVA is great